3 is clearly the favorite but the way this is set up puts 3 at a strategic disadvantage.
4 would join 5 just for survival or 5 would take 4 pretty quickly. Same for 2 with 1.
So it'll eventually be 4+5 vs. 2+1 vs. 3.
In that case 3 is STILL the heavy favorite so fully expect 1+2 and 4+5 to look at 3 as its main threat. 3 would essentially have to fight a two front war. That would bleed 3 out of its most valuable resource (a dominant fighting force). 3 would have to defend the carolinas, west tx, and ok/ks to prevent 1+2 and 4+5 from getting to dallas, atlanta, houston, miami, etc.
After a long drawn out attrition, 3 would probably be done. 4+5 would have the advantage over 1+2 solely due to money, climate, resources, and logistics.
So the answer is 5 if they could team up. If they remained seperate, 3 would wash everybody pretty easily.