Thanks for your answers guys, all your advice is welcome.
try to use all identical cards.i'll go for a 1070, if you sell your 3 750Ti, that should pay for about 50% of what a 1070 costs.
that's what i did, i sell all my 750ti's (i had like 30
) and buy 1070's my power bill decreased like 30% 750ti where fantastic cards, the best ones i ever used, but their time for mining is over IMHO.