Hello zisisfadeout,
as far as I remember, you couldn't see the enemy mob/player HP in Vanilla anyway, not even with addons. Now in combination with MobHealth3, you can see the mobs HP but not the players. As I was using LunaUnitFrame (can be found in this Forum) until recently, after defeating a Player/Mob, the Addon actually calculates the damage done to the target and estimates his HP most of the time so I was fairly satisfied with that. Now I'm using the Ohriginal UI (also on the Forums) that also uses LunaUnitFrames and I have yet to find out if it does the same job (looks neat though
).
TL;DR: Give LunaUnitFrames a try.