Three devices clustered but only 2 is going active when using "force to standby"

Yes, that is indeed how it's supposed to work. The default failover method for a traffic group is "Load Aware" so unless you have multiple traffic groups and bigip1 is already active for those I suspect that since there is a tie between bigip1 and bigip3 when you force bigip2 to standby bigip1 will win simply by being the first BIG-IP in the list. Read more about the Load Aware calculation here:

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/tmos-implementations-11-6-0/6.html?sr=49108538