Hi Kevin,
What you describe sounds correct. You should be able to make this work by configuring HUB3 as a hub for all your spoke routers, and making HUB3 a client of HUB1 and HUB2.
You’ll need EIGRP add path support on all hub routers:
Without it, your hub routers will only have a single route for the network(s) behinds SPOKE1 + SPOKE2.