This phenomenon, called "substrate inhibition," is pretty common. It is described quantitatively by the substrate inhibition constant (Kis). In the simplest case of a one substrate enzyme, the Michaelis-Menten equation is modified as follows:

V = Vmax[S]/{Km + [S](1+[S]/Kis)}

Note that this results in an [S]^2 term, which is why you see the rate increase and then decrease again.

from what i understand, i have to determine Vmax, KM, and Kis for each case

Yes.

i don't know if what i did is right but i calculated Ki just after Vmax ( where rate started to decrease  ) i got pretty similar Ki values ! ( Ki ~ 0.102043)

is this anywhere near right ?

sorry any inconvenience  i have no experience with biology its all new to me

Looking at your graph, I would expect the value of Kis for A, S, and N to be in the single-digit or double-digit range, on the x-axis scale you used. 0.1 looks too low. Did you use nonlinear regression to the modified Michaelis-Menten equation?

By the way, you may want to check the correctness of the units on the x-axis. They should be concentration units (e.g.moles/liter, not moles/liter2). And were the substrates really at molar concentrations?

- this time i did use nonlinear fitting using the Michaelis-Menten equation (graph in the attachments ) but after the fitting converged i got a higher Vmax then the peaks of the plots ! is that normal ?

- units on the X-axis are correct, yes the were really molar concentrations

Nice work. Yes, it is expected that Vmax will be higher than the peak rate, due to the fact that inhibition is occurring even at the substrate concentration that gives the peak rate.

Do Km and Ki Values look good ?

Your estimates would benefit from testing concentrations between 0 and 1 M, where the most of the rise in rate with concentration is occurring, in addition to the higher concentrations.