Question: plot3d array function

The fubction I've got  for ex:

f(x,y)= sum(exp(x*r[i])*a(r[i]),i=1..3):

where:

r[1],r[2],r[3] are functionsof y

I plot f in 3d in the case when r[i] are values. no problem

but i got confused when r[i] are functions

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