A/c wiring

I'm installing a central ac unit and I was gonna run red black and a ground in flex from 25 amp hacr breaker to fuse less disconnect then a whip to unit outside. I was going to use 12 guage. I was checking the book again and it says recomended wire size is 12-3. My question is the neutral is useless right? Would I cap it on both ends and leave it be if I did it this way? Or am I missing something?