|You didn't say what you wanted to do with the exchange rate, so this formula will simply return it.|
Assuming your 1, 2, 3 are in Column A and your exchange rates are in Column B:D, try this:
Note: This formula will return D1 if A1 is anything except 1 or 2.
If it were me, I'd use a Data Validation Drop Down in A1 to ensure that only a 1, 2 or 3 were entered.
You could also use something like this to ensure a 1,2 or 3 is entered:
=IF(AND(A1<>1,A1<>2,A1<>3),"Enter a 1, 2 or 3 in A1",IF(A1=1,B1,IF(A1=2,C1,D1)))
If you need to actually do something with the exchange rate, like multiply a value from another cell with it, something like this should work.
Click Here Before Posting Data or VBA Code ---> How To Post Data or Code.