Connacht aiming to do an unlikely double over four-times champions Toulouse
The Irish teams will be looking to back up their clean sweep last week when the second half of the European Cup double-headers get underway this weekend.
All four Irish provinces were on the winning side last week, and the challenge now is for them to repeat the feat against the same opposition at different venues in Round Four.
Leinster and Connacht will play at home this weekend, with lowly Connacht aiming to do an unlikely double over four-times champions Toulouse, whilst Munster and Ulster will have to travel to Perpignan and Treviso respectively.
Glasgow Warriors had hoped to be cementing their push for quarterfinal qualification when they welcome Cardiff Blues to Scotstoun on Friday but instead it is the visitors who are in pole position to challenge Toulon for top spot in Pool Two.
Glasgow headed to Cardiff last week knowing that victory in the Welsh capital would have seen them sitting pretty at the halfway stage of the campaign and with two home games still to come.
Defeat at the Arms Park has left them with at the foot of the table, though, and they now face an uphill struggle to reach the last eight given that Toulon won at Exeter on the same weekend.
Gregor Townsend's men failed to even pick up a losing bonus point in Round Three, meaning a maximum point haul on home soil is now even more important to their hopes of a maiden quarterfinal.
As for the Welsh outfit, Phil Davies' troops are on a major high after receiving rave reviews for their performance last time out. Victory over holders Toulon in Round Two was a huge result for the region but they still weren't expected to back it up given that they sit ninth in the Pro12, some 13 points behind third-placed Glasgow thanks to just three league wins all season.
But back it up they did, and in some style, too. The Blues bounced back from a 31-6 mauling by Munster in their previous outing at the Arms Park despite missing a host of big names through injury, including British and Irish Lions skipper Sam Warburton. Warburton won't be fit to feature on Friday, either, as he is awaiting specialist advice on a 'stinger' problem with his shoulder.
Lions wing Alex Cuthbert marked his return from ankle damage with a well-taken try last week but the Blues' injury crisis has since deepened with the news that centre Dafydd Hewitt won't be available for selection. Hewitt becomes the fourth midfielder to be ruled out, with Owen Williams, Cory Allen and Gavin Evans already out of action.
All the teams and match details:
Friday, December 13
Ospreys v Castres
(Liberty Stadium - Kick-off: 20.00; 20.00 GMT)
Ospreys: Ospreys: 15 Sam Davies, 14 Aisea Natoga, 13 Tom Isaacs, 12 Ben John, 11 Jeff Hassler, 10 Dan Biggar, 9 Rhys Webb, 8 Ryan Jones (captain), 7 Justin Tipuric, 6 Sam Lewis, 5 James King, 4 Ian Evans, 3 Aaron Jarvis, 2 Richard Hibbard, 1 Ryan Bevington.
Replacements: 16 Scott Baldwin, 17 Marc Thomas, 18 Dan Suter, 19 Tyler Ardron , 20 Morgan Allen, 21 Tom Habberfield, 22 Matthew Morgan, 23 Tom Grabham.
Castres: 15 Geoffrey Palis, 14 Marcel Garvey, 13 Remi Lamerat, 12 Paul Bonnefond, 11 Remi Grosso, 10 Remi Tales (captain), 9 Rory Kockott, 8 Antonie Claassen 7 Yannick Caballero, 6 Jannie Bornman, 5 Rodrigo Capo Ortega, 4 Richie Gray, 3 Anton Peikrishvili, 2 Marc-Antoine Rallier, 1 Yannick Forestier.
Replacements: 16 Brice Mach, 17 Saimone Taumoepeau, 18 Michael Coetzee, 19 Christophe Samson, 20 Ibrahim Diarra, 21 Julien Tomas, 22 Daniel Kirkpatrick, 23 Max Evans.
Referee: Luke Pearce (England)
Assistant referees: Dean Richards (England), Robin Goodliffe (England)
TMO: Geoff Warren (England)
Glasgow Warriors v Cardiff Blues
(Scotstoun Stadium - Kick-off: 20.00; 20.00 GMT)
Glasgow: 15 Stuart Hogg, 14 Sean Maitland, 13 Byron McGuigan, 12 Alex Dunbar, 11 Tommy Seymour, 10 Ruaridh Jackson, 9 Niko Matawalu, 8 Ryan Wilson, 7 Tyrone Holmes, 6 Rob Harley, 5 Al Kellock (captain), 4 Leone Nakarawa, 3 Jon Welsh, 2 Pat MacArthur, 1 Ryan Grant.
Replacements: 16 Dougie Hall, 17 Jerry Yanuyanutawa, 18 Ed Kalman, 19 Josh Strauss, 20 Chris Fusaro, 21 Chris Cusiter, 22 Duncan Weir, 23 DTH van der Merwe.
Cardiff Blues: 15 Leigh Halfpenny, 14 Alex Cuthbert, 13 Richard Smith, 12 Rhys Patchell, 11 Harry Robinson, 10 Gareth Davies, 9 Lloyd Williams, 8 Robin Copeland, 7 Rory Watts-Jones, 6 Macauley Cook, 5 Filo Paulo, 4 Chris Dicomidis, 3 Taufa'ao Filise, 2 Kristian Dacey, 1 Sam Hobbs (captain).
Replacements: 16 Marc Breeze, 17 Thomas Davies, 18 Benoit Bourrust, 19 James Down, 20 Ellis Jenkins, 21 Andries Pretorius, 22 Lewis Jones, 23 Dan Fish.
Referee: Pascal Gauzère (France)
Assistant referees: Laurent Cardona (France), Jean-Luc Rebollal (France)
TMO: Philippe Bonhoure (France)
Saturday, December 14
Toulon v Exeter Chiefs
(Stade Felix Mayol - Kick-off: 14.35; 13.35 GMT)
Exeter Chiefs: TBC
Referee: John Lacey (Ireland)
Assistant referees: David Wilkinson (Ireland), Michael Black (Ireland)
TMO: Dermot Moloney (Ireland)
Treviso v Ulster
(Stadio Comunale di Monigo - Kick-off: 14.35; 13.35 GMT)
Referee: Greg Garner (England)
Assistant referees: Steve Lee (England), Roy Maybank (England)
TMO: Trevor Fisher (England)
Scarlets v Clermont Auvergne
(Parc y Scarlets - Kick-off: 15.40; 15.40 GMT)
Referee: Wayne Barnes (England)
Assistant referees: Matthew Carley (England), Nigel Carrick (England)
TMO: David Grashoff (England)
Saracens v Zebre
(Allianz Park - Kick-off: 15.00; 15.00 GMT)
Referee: Dudley Phillips (Ireland)
Assistant referees: Peter Fitzgibbon (Ireland), Kieran Barry (Ireland)
TMO: Alan Rogan (Ireland)
Perpignan v Munster
(Stade Aime Giral - Kick-off: 16.40; 15.40 GMT)
Referee: JP Doyle (England)
Assistant referees: Martin Fox (England), Paul Dix (England)
TMO: Rowan Kitt (England)
Leinster v Northampton Saints
(Aviva Stadium - Kick-off: 18.00; 18.00 GMT)
Northampton Saints: TBC
Referee: Jérôme Garcès (France)
Assistant referees: Sébastien Clouté (France), Patrick Pechambert (France)
TMO: Eric Gauzins (France)
Connacht v Toulouse
(The Sportsground - Kick-off: 18.00; 18.00 GMT)
Referee: Leighton Hodges (Wales)
Assistant referees: Gwyn Morris (Wales), Chris Williams (Wales)
TMO: Paul Adams (Wales)
Sunday, December 15
Harlequins v Racing Metro
(Twickenham Stoop - Kick-off: 12.45; 12.45 GMT)
Racing Metro: TBC
Referee: Alain Rolland (Ireland)
Assistant referees: Sean Gallagher (Ireland), Gary Conway (Ireland)
TMO: Simon McDowell (Ireland)
Gloucester v Edinburgh
(Kingsholm - Kick-off: 15.00; 15.00 GMT)
Referee: Romain Poite (France)
Assistant referees: Cyril Lafon (France), Stéphane Boyer (France)
TMO: Jean-Pierre Pellaprat (France)
Montpellier v Leicester Tigers
(Stade Yves du Manoir - Kick-off: 16.00; 15.00 GMT)
Leicester Tigers: TBC
Referee: Nigel Owens (Wales)
Assistant referees: Sean Brickell (Wales), Wayne Davies (Wales)
TMO: Derek Bevan (Wales)