>=FunctionX(Param1,Param2,Param3) and an error occurs.
>Do you know how to call it?

The VBA function syntax is a little non-intuitive from a VFP viewpoint:

Function FuncX(a, b, c)
FuncX = a + b + c
End Function

cell A1:
=FuncX(1,2,3)
yields 6

Assigning the result to the name of the function is less intuitive than Return (result)