blow off safety valve third stage opens
Safety valve for third stage is blowing off
Holy cow, Victor, you really don't provide a whole lot of information, do you?
Assuming that the compressor is working properly, that the safety valve in the third stage is working properly, then it's blow off because the air pressure in that stage is exceeding the pressure setting for the safety (PRV) valve.
If the third stage is going into the tank, then maybe, just maybe (this is just a guess), the check valve going into the tank has a problem with not opening properly, and the back pressure is causing an air pressure spike in that stage, blowing the PRV.
You need to monitor the pressure throughout, to be able to determine where and when the spike occurs.
Wish I could be of more help, but I've really got no information to go on.
Cheers and good luck.