Serial3 is by default hooked up to the old D13 and D14 pins (when a DXL MKR Shield was needed) and it is working fine.
Adding Serial4 and Serial5 is also possible using the SERCOM approach shown at this post, using (D0, D1) and (D2, D3).
So, plenty of COM Ports to talk to other Controllers, PCs and SBCs with the OpenRB-150.