Would you be drinking it 3 times a day or just once? If just once, I would say directly after the work-out (anyone, feel free to correct me if I am wrong). Drinking the shake on non-gym days can't hurt, it's just regular shit that your body needs. It doesn't really matter if you skip a day, either. I drink mine Mon-Sat. and just skip Sunday (when I do not work out). It's all up to you, basically, not much of a difference what you choose.
And again, feel free to correct me hahaha, this is just what I have gathered from people.