DHCP on 3 Sector AP?

I am contemplating a Mikrotik AP with 3 Cards (we run this setup alot already). However I would like to have each sector (total of 3) to have it’s own IP space. The setup will consist of an AP with 3 sectors on the tower and a routerboard at the bottom. The routerboard will be doing DHCP, shaping,etc. I do not wish to do DHCP at the AP level unless it is just relay or something to the routerboard at the bottom. My question is this:

1.How can I seperate the IP space? I have found you can’t run DHCP servers on VLAN interfaces.

How is everyone doing this?

Thanks,
Justin