r there 3 formulas for average velocity ? [s/t] , [u+v/2] and [displacement upon time] ?

Yes, av. velocity = ( u + v ) /2 = displacement /time .

But in all cases we cannot take s/t as the formula for av. velocity as here 's' means distance and not displacement. Also  in questions , where the distance is given to be  in a particulr direction, this formula can be used.