How about using two fields of a primary index containing three fields?

I'd like to select from a table called "MMStdPickingOrderLine" by referencing on two fields (custAccount and pickingOrderId). This table has actual only one index which is primary and contains 3 fields. the fields I use in my statement and a third one. Will this Index work properly if I only use two of three fields or will I have to create a new index?

Thanks a lot.
René Pohlmeyer