Here are the 4 possible combinations of wiring you may have (if power from breaker is coming from your light then this is not possible). The two switches on the left would be your existing 3 way switches while the one on the right is the single switch. The first light is the one controlled by the 3 way, the second the one controlled by the single switch. These diagrams show the new 3-wire line you would need to add.
