Maybe it's just the only way I learned to play, but armored core 3 I found only the floating legs were very viable. Then I would equip self aimed missiles and found arena quite simple. The missions you could use different set ups, but I could never win arena much with any other legs.
In 2 and (I think) 3, there was a fifth class of legs called "hover" that basically acted like quads do in hover mode in AC6, but bound to the ground unless you boosted. IIRC, they had the easiest time getting airborne from a boost, but had no unboosted jump. Basically, they were faster treads that could move sideways.