The trio's availability is a timely boost to Ulster after their frustrating 23-22 Magners League loss to the Ospreys last Sunday.
Trimble, Wallace and Court will no doubt be looking for a big performance to force their way into Ireland team for next week's Championship clash with Scotland.
Ireland Under-20 out-half Paddy Jackson has been added to the panel and will be hoping to get his first taste of Magners League action in Llanelli, having been an unused replacement against Munster last month.
Ian Humphreys is also included despite being forced to leave the pitch early on against the Ospreys with a shoulder injury.
Both Ulster and the Scarlets will see Friday's game as a 'must win' fixture if they are to realise their ambitions of a top four finish.
The Scarlets have lost their previous five matches in all competitions, but despite this poor form they currently occupy fourth position, two places ahead of Ulster.
Defeat for Ulster would see them lose further ground on their rivals so they will be intent on producing an improved performance. The province's matchday squad is set to be announced on Thursday.
ULSTER Squad (v Scarlets) - Backs (16): Ruan Pienaar, Paul Marshall, Ian Humphreys, Niall O'Connor, Paddy Jackson, Ian Whitten, Nevin Spence, Paddy Wallace, Luke Marshall, Andrew Trimble, Simon Danielli, Tommy Seymour, Craig Gilroy, David McIlwaine, Jamie Smith, Adam D'Arcy.
Forwards (16): Jerry Cronin, Declan Fitzpatrick, Paddy McAllister, Bryan Young, Tom Court, Nigel Brady, Andi Kyriacou, Ryan Caldwell, Neil McComb, Johann Muller, Tim Barker, Pedrie Wannenburg, Thomas Anderson, Willie Faloon, Chris Henry, Robbie Diack.