Ricky Andrew, who started for Ulster against Zebre last weekend, is named at full-back and is joined by Peter Nelson and Ballynahinch's Ross Adair in the back-three.
Dungannon's Stuart McCloskey is selected alongside Wallace in the centre, with Michael Heaney at scrum half. James McKinney, fresh from his man-of-the-match performance for Ulster against Edinburgh, is named at out-half.
Up front, Hughes Insurance Academy players Kyle McCall and Jonny Murphy are joined by Ricky Lutton in the front row. Queen's University clubman Marc Campbell is named in the second row along with Matthew Rea.
Ireland Under-20 flanker Conor Joyce is selected in the back row as is Frank Taggart of Belfast Harlequins and Ballymena's Adrian Hamilton.
Adam Macklin, who is also back after an ACL injury, is named on the replacements bench as is hooker Paul Jackson, older brother of Paddy.
Scrum half David Shanahan, who made his debut for Ulster against Edinburgh a fortnight ago, is also selected as a replacement.
Ulster Ravens have played two matches so far in the competition, losing to Cornish Pirates in the first round before beating Jersey at Ravenhill in round 2.
ULSTER RAVENS (v Ayr): Ricky Andrew; Ross Adair, Stuart McCloskey, Paddy Wallace (capt), Peter Nelson; James McKinney, Michael Heaney; Kyle McCall, Jonny Murphy, Ricky Lutton, Marc Campbell, Matthew Rea, Conor Joyce, Frank Taggart, Adrian Hamilton.
Replacements: Paul Jackson, Adam Macklin, Josh Atkinson, Lorcan Dow, David Shanahan, Darren Busby, Adam Liddell.