which sanji should i give him skill lv up?

hi

i have 1 low sanji, 2x mid sanji which i want one evolve to diablo sanji.. and i have 1 mr prince sanji.. i want give a sanji my low sanji and a mid sanji to a sanji to give him a skill lv up, so which sanji should i give? diablo or mr prince?

You should keep Mr Prince because he is a weaker version of Gear 3 Ruffy. And if you evolve Sanji to Diable Jambe his special will change so that an other Sanji won't make his Special Level up. And Mr Prince's special is also another one so you also can't make his special level up with the two lower sanjis.
To sum up: Evolve one Sanji to Diable Jambe and keep Mr Prince. With the other two sanjis you can do what you want. 
PS: I don't know if I get the question right but I hope so

i mean which sanji should i give a skill lv up

Skill Up Mr Prince Special is better because Sanji Diable Special have only 15 Turns. ^^

ah, i see now in the next event isle you can drop mr prince skillbooks.... o: what now?

As mr prince skillbooks only work on him because of the skill name, you should focus on reducing his cooldown/ maxing out his special level. The books won't work on the other Sanji.

should i then give it to mr prince and then evolve?

If you want a special level up you have to give the mr prince skillbooks the evolved mr prince, because the skillbooks only work on the evolution because of the skill name