As far as I can tell, the two places where the Great Minds that Think Alike are referenced are here (http://www.erfworld.com/wiki/index.php/LIAB_Text_38) and here (http://www.erfworld.com/wiki/index.php/LIAB_Text_40).
In the first one, the only thing said is that "It was a coded message, passive, and costing no juice. Any Thinkamancer who plucked her would find this note in her G-String. But hopefully, only one of the Great Minds that Think Alike in the Temple of the Thinkamancers could decode it. "
It means that the Great Minds are in the Temple, but says nothing of how many or what they do there or who they are.
In the second one, however, we can I think glean more useful information. Isaac the thinkamancer says "A few of the Great Minds lingered nearby, occasionally using the equipment with hesitation. But Isaac was aware that this place usually stood empty when he was not here. He himself was the attraction, ". ... "If only Charlie were one of the Great Minds. If only he Thought Alike. "
It says that the Great Minds can be outside with the equipment, and not just in the temple. That they can afford to be attracted to Isaac when he's around, and not there when he isn't. That "a few" of them can be around.
To me, that indicates that The Great Minds aren't a tri-link. They may include some of their members in a tri-link, but that's not the same thing.
Casters in a tri-link would be much, much more hesitant to wander around and distract themselves just because Isaac was around. It would make little sense to describe "a few" casters of a tri-link going somewhere - when there's only three to begin with, it's either one or two or three that could be wandering around.
To me, that's the biggest indication that it's not a tri-mancer link. If there's only three Great Minds, as in a tri-link, then it wouldn't make sense to say "a few of..."
Also, we know that Isaac EASILY decoded Maggie's message - and that Maggie said that "hopefully, only one of the Great Minds that Think Alike" could decode it. We should expect that Maggie knows that Isaac would easily decode it, or at least suspect it. She didn't seem to mind that Isaac decoded it, expressing no surprise when he later contacted her by thinkagram. Thus, if Maggie believes that only one of the Great Minds could decode it, and Isaac decoded it easily, and Maggie was not surprised by this, that implies that Isaac is one of the Great Minds.