Only dove in a "nominal" triple once, my local stomping grounds at A Bay.
In that case I asked my buddy to buddy with the third person. The other person had been diving for years, but not recently, so this was intended as a shallow, short distance, see the sights activity, within 75' of shore and max depth 50' -- solid bottom, no chance of someone just dropping down.
For myself, it was considered a solo, that's all there was to it, neither of the others expected to watch my back. Had my pony, of course.
It worked well. They were a good buddy pair, I generally watched both to make sure nothing was going weird but otherwise was keeping an eye on myself and just enjoying a nice shallow dive. We were up at 750 psi.
If there was a whoops, multiple other groups were diving, though not too crowded, and I would hopefully have done as others suggested -- raise an alarm, let the folks with more training and gas, and less nitrogen, do what they know how to.
I know that at least in concept a triple can be done, but my belief is that to be a "good" mechanism the three need to be really solid, in sync, folks who regularly do dive together.
(Rescue Diver really is an excellent class. Also, I'm working towards DM in part because that gives many opporunities to learn, and relearn, fundamentals, chains of thought that I want to be automatic. My Instructor keeps drilling the "don't become another victim" concept. I'm a thick-headed Irishman -- need to keep hearing the same thing, practicing it, in order to expect to actually do it when the need arises.)
Looking forward to the Keys