Some lights are being feed off 20amp breaker. Can you use 14 gage wire in this circuit to add 3 way switches?

No, if you do, you need to change breaker to 15 Amps.

Thanks for prompt reply. That's what I thought and planned but just wanted to be sure I had to do that..
Many thanks again.

No if you wish to be code compliant. What is the load on the circuit? If a 15 amp breaker will be no problem change the breaker ( $4 ).

I need to add another 15amp to break this circuit down into 2. It currently has 6 HO 8' Fluorescents that draw about 13.5 amps. I think that will still be cheaper and allow me to run 14gage 3 way wiring. There are 6 switches in this application. New wire runs to add 3 Way switching will be 40 feet for each of the 6 switches. THanks for your help.

Each lamp is switched. Are they normally all on? If not there is no reason to split the circuit. Even if they are all on they will not trip a 15 amp breaker as long as there is nothing else on the circuit. It they are all on for more then three hours they are considered a continuous load and to be code compliant the circuit should be split. As a practical matter a 15 amp circuit is okay.