From the manual:
Switching P25Reflector servers using your radio

You can switch P25Reflector servers with your radio by calling the desired server's ID with a private call. If the Allow server change with group calls option is enabled (it is enabled by default), then starting a group call to the desired talkgroup's ID performs a server switch. I run a P25 network along with the DVSwitch P25 Reflector, Gateway and Parrot. I've owned SharkRF Products since the early days of the OS1. Having just received my OS3 and with vacation in the not too distant future, I've been testing the OS3's P25 features. Everything is working great except for P25 TG/REF Steering.

As per the above quote, I've tried with both group calls and private calls to change the reflector without success. Here is a screen shot showing the OS3 recognized both call types along with the destination TG/REF, but did not change from the currently connected TG/REF.

Pic (Attached) #1: Showing connected to TG/REF #6395
Pic (Attached) #2: Showing receipt of group call to TG/REF #31665 but still connected to TG/REF #6395
Pic (Attached) #3: Showing receipt of private call to TG/REF #31665 but still connected to TG/REF #6395

Radios used to test: aligned Motorola APX7000 and an aligned Motorola APX8000. Same results.

Any thoughts?