Don’t get fooled by the wording: PrefNic does NOT equal “preferred” NIC, but also use others.
Meaning: setting the DRBD Replication communication to this NIC first, if some (the majority?) nodes can access the network behind the NIC and communicate with each other, they will do. Other nodes not connected to this network - you mentioned a cross cable! - won’t take part in the DRBD Replication communication.
A DRBD quorum (you might use your 3rd node for this to get HA features with a tie-breaker in LINSTOR / DRBD) does need to take part in replication communication or won’t be a functioning quorum with all negative effects on the HA features you’d love to have.
See my annotation in Proxmox: no DRBD storage while LINBIT controller is down - #19 by proxmeup
I actually wanted to have already moulded the topic of PrefNic / fallback NIC into a question…