Assume your USD are in cells A3:A10
Your GBP are in B3:B20
Your EURO are in C3:C15

Your conversion from GBP to USD is in B1
Your conversion from EURO to USD is in C1

Then try:
=sum(A3:A10,B3:B20*B1,C3:C15*C1)
After entering the formula, hit ctrl-shift-enter instead of just enter.

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