I believe anymore, the 3 ton floor jacks are the bare minimum for a heavy (say, 4000lb plus) vehicle. The lighter ones have a very small "cup" for lack of better term, that the vehicle is actually in contact with. The larger jacks have about a 4-5" piece, and the cheaper ones are more like 3-4".
Also, the whole jack is beefier, the lighter duty ones you can see them "sway" side to side under a lot of weight, and personally, thats scary. Expect to pay no less than $60 for a 3 ton jack from what I've seen lately. I got mine at one of those traveling tool shows (lol) and it was $60, seems to work fine. The cheapest I've seen them in local stores is about $80-90.