Hello John
Yes, Cisco does support three hubs in a single cloud design. Indeed, it supports even more than three hubs if your network design demands it.
DMVPN supports multiple hubs, and the number and arrangement of the hubs depend upon the geographical dispersion of the specific network and its size. For extensively large networks, hubs can be arranged in a hierarchical or tiered structure to optimize traffic flow and management.
DMVPN has been designed to be highly scalable, and this of course means that you can indeed have three or even more hubs.
I hope this has been helpful!
Laz