What are the best places to level up all three characters?

Keyblade Graveyard.

Okay. I haven't unlocked it for aqua I just got to enchanted dominion.

The best place is , at least on my opinion, those tornados in Keyblade graveyard. In fact, that is the best because of the number of Exp given , not the number of unversed.

So that's the bet place to level up the HP

well if you get both emblems for the mickey d-link, you will receive the double experience ability for that d-link, which is really helpful, im not sure if it is like the kh2 one where you must be in critial health, but i doubt it

HP? Well, you can get 3 HP Boost abilities, the easiest way I've found to do that is to fuse two Thunders with a Soothing Crystal. Several Mirage Arena fights give an HP bonus (It varies by character)

Beyond that, the only way to increase your HP is progressing the story (The Second to Final Battle gives an HP bonus, so you can't have Max health until you beat the characters story)

Rubberband + KHBBS + PSP + EXP walker ability = Around 18k an hour

Aqua: Keyblade Graveyard

Terra: Keyblade Graveyard

Ven: If you are at a lower level (at around level 1-15) then train at the huge room at Castle of Dreams where you fight Luficer. If you are at a higher level (16-99) then train at the Keyblade Graveyard

For terra, I find enchanted dominion a good place to train cause I haven't unlocked Keyblade Graveyard yet, about one and a half times through and you level up. But I'm struggleing with the hp too, in the guide it shows terra with crazy-high hp but I can't really get there. By the way I'm level 30 right now so the first one works when you're around this level or lower, I'm not sure about higher.

For terra, I find enchanted dominion a good place to train cause I haven't unlocked Keyblade Graveyard yet, about one and a half times through and you level up. But I'm struggleing with the hp too, in the guide it shows terra with crazy-high hp but I can't really get there. By the way I'm level 30 right now so the first one works when you're around this level or lower, I'm not sure about higher.

 

You an't gain HP from leveling up. There are 3 ways to gain HP.

Beat story events, such as bosses (The second to last Final Boss gives an HP bonus in every story, so you won't have max health until you beat the game)

Get the HP Boost ability. You can equip 3 of these and they net you 8 HP each. The easiest way to get them is to meld two thunders and a Soothing Crystal

The Mirage Arena. Completing events here sometimes nets you HP bonuses (Among other things)