Hello every1!

Can anyone devise me a piece of code to formulate a custom function like:
MyRate(Array1,Array2,Array3,Criteria)

To work for a function like:
=SUMPRODUCT(--(A1:A20="A"),B1:B20,C1:C20)/SUMPRODUCT(--(A1:A20="A"),B1:B20)

As:
MyRate(A1:A20,B1:B20,C1:C20,"A")

All your assistance and expertise shall be highly obliged.

Thanx in advance!!!