You can not add function handles together. Instead, you can add the integrals.
format long
f=0;
for t=1:5
%I am assuming you want to do multiplication by using the notation xyz
fun = @(x,y,z) t.*exp(x.*y.*z);
%Adding integral values
f = f + integral3(fun,0,1,0,1,0,1);
end
f
