Snubber circuit for 3 phase inverter designed for BLDC motor
Firstly, try to mount RC snubber in parallel with transistors.
You can choose values experimentally (f.e. 1k resistor, 1nF capacitor). 
It's difficult to calculate correct values (probably you don't have data about circuit inductance). 
Please note to apply resistor with correct power (depend on frequency, capacitor value and switching current) and capacitor with appropriate withstand voltage (including spikes). 

http://www.cde.com/resources/catalogs/igbtAPPguide.pdf
http://www.illinoiscapacitor.com/pdf/Papers/RC_snubber.pdf