I_Cheat_U_LOSE
Sucks 2 Be U
Jill and Chris most likely win.
Leon has better martial arts skills and agility, and has very good judgement
Chris is better with firearms and is stronger
Jill (RE5) is trained in explosives and in RE5 is very agile and fast
Ada is best working in the shadows, saying hidden, and being in the background
Considering that Chris and Jill have much more experience working together ex: both members of Stars, Umbella Chronicles, etc they are more likely to work as a team. While with Leon and Ada, it will mainly be Leon trying to fight against Chris and Jill while Ada works behind the scenes trying to assist Leon. Also since RE is a game where guns are the primary weapons and its guns that kill; its makes Chris' firearms skills more vital than Leon's agility and good judgment.
Most Chris and Jill will fight together and Leon will be on the ground while Ada assist afar. Thus the likely scenario is that Leon will get overwhelmed by Chris and Jill double-teaming him. Ada tries to assist Leon but isn't fast enough, but at least wounds Chris or Jill in the process. However, she gives away her location and then gets double-teamed by Chris and Jill.
Leon and Ada can win this if Leon is able to stall out Chris and Jill, while Ada works on releasing a new BOW to distract Chris and Jill from pursuing Leon. Leon then fights Chris hand to hand when Chris is out of ammo, and Ada snipes Jill when she tries to assist Chris.