Make a field manditory based on a non-manditory field??

Make a field manditory based on a non-manditory field??

All:
 
I'm looking in Service Desk to have a Incident Template have Field A as non-manditory but if Field A is populated, then Field B and Field C are now marked as Manditory.  Does anyone have a recommendation as to how to accomplish this?
 
We are using Service Desk Plus 8.1.0 (build 8121)
 
Thank you in advance.

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