I agree with konnichiwa, you should make it any game over 80%. Assuming most people would consider >=80% "good".
Example scenario (all figures in %):
Month1 Month2 Month3
PS3 89,60 84,68,64 90
Wii 86,55,62 88,60 87,70,64
360 80,84 84,82,72 85,80,81
Based on your current rules - PS3: 2, Wii: 1, 360: 0.
One could successfully argue that 360 had more good games released over the 3 month period. In fact, it had more than the other 2 consoles combined, yet it scored 0.
I also agree with The_vagabond7 - most of this is just cherry picking.