Ask - Better way to connect 3 Networks

Hi there,
need advice from expert here.

we have built networks between branch and Headquarter using mikrotik hardware.

HQ (192.168.1.X) <—> Branch ( 192.168.2.X)

then, at HQ, we have to connect to building management’s network (BM), and there is service on BM that we use, therefore the network become:
BM (192.168.100.X) — BM’s Router Firewall --<-------> HQ (192.168.1.X) <—> Branch ( 192.168.2.X)

Now, we want branch also can use service on BM.
how to I setup, so Branch can connect to service on BM network?
the only possible access is Branch communicate over HQ network, then to BM’s Network.

thank you.