The Ospreys travel to the RDS to face champions Leinster who are protecting a record of only one defeat in their last 27 games in Dublin.
The third undefeated side, Munster, are coming off the back of their big win over the Newport Gwent Dragons last weekend with another home game against Welsh opposition - they entertain the Cardiff Blues on Saturday evening.
Elsewhere, two teams without a win meet on Friday night when the Dragons visit Ireland again, this time to face Ulster in Belfast.
The final fixture, also on Friday night, pits the Glasgow Warriors against Connacht, who are looking for their first points in the league so far this season, in Galway.
MAGNERS LEAGUE - ROUND 3 FIXTURES:
Friday, September 19 -
Connacht v Glasgow Warriors, The Sportsground, 6.30pm
Edinburgh v Scarlets, Murrayfield, 7.30pm
Leinster v Ospreys, RDS, 7.35pm (live Setanta Sports Ireland/BBC Wales)
Ulster v Newport Gwent Dragons, Ravenhill, 7.30pm
Saturday, September 20 -
Munster v Cardiff Blues, Musgrave Park, 7.30pm (live Setanta Sports Ireland/S4C)