Complex HA scenario with 3 Big IPs

HA Order uses all units in a specific order.So, if a choose only LTM_1 and LTM_2, when both of them are down then LTM_3 becomes active.

not sure if this is possible. can we configure ha group but preventing LTM_3 to participate in failover (e.g. fake pool, fake trunk)?

Release Note: BIG-IP LTM and TMOS 11.5.0 (HA group failover for traffic groups)

http://support.f5.com/kb/en-us/products/big-ip_ltm/releasenotes/product/relnote-ltm-11-5-0.html

by the way, there is request for enhancement but it is not implemented in the current version yet. you may open a case to see if you can expedite it.

ID403789 - RFE: Allow creation of traffic group with a subset of the devices in the sync-failover device group