That seems underrated to me. 3mAh * 20c = 60A rated discharge rate It's a 500 size heli? So it is likely pulling 50-80A at peak, depending on the motor you're using? Probably has a 60-80A ESC on it?
It may work for light flying, but if you're running idle up for extended periods and doing any kind of sport flying I'd spring for 30-35c (maybe 40-45c if I'm running a high current motor) so I wasn't stressing the batteries at all during flight. They'll sag less at the end, less risk of running into the low voltage slow down during flight, come down cooler, have a longer life, etc. To me it would be worth it.