Those talkgroups are patched together at times when there is only one dispatcher to cover multiple channels. the TG+3 is the system's way of marking that talkgroup as being patched.
You never can tell which talkgroup will be the "supergroup" and which other talkgroup or groups are patched with it. So sometimes you might see 52179, and other times you might see 7539, and other times you might see 7667. So it's best to have all of those talkgroups programmed so you can be sure to hear all the traffic.
And, as Larry says, if you use a GRE scanner or set the Status Bit status to ignore, you don't have to worry about it.