Because you put the 1st two in together, they adapted the tank and the situation they were forced into, as their joint territory, and may continue as such, for the 2 of them (temporarily).
Any new cichlid (or cichlid-like fish) you add now in that space, will be seen as an interloper, and threat to the tanks territorial integrity, to be vanquished.
There is possibility that if you get a 125, set it up, and add all 3 at exactly the same time, that they then will set the territorial space as something to be broken up as room for 3. There is """also" a 50/50 chance that they won't.
And.......but..........all cichlids change with time....go thru territorial phases with the onset of maturity....so just because it works one day, does not mean in will work in the future.
In nature a mature 10" JD will hold a territory of equivalent about 250 gallons, and chase any other cichlid (except a complaint female) out of it.
In the video below , at about the 55 second mark, you can see a territorial JD defend a territory of about 6 ft L by 6 ft W, by 6 ft in depth from any other cichlid near a paper marker. It instinctually chases away any other JD except his chosen female, you may also notice he doesn't bother with the non-cichlids, not considering them a territorial threat. Cichlids are extremely concerned with available resources available, any other cichlid also needs.
JDs and Catfish of Cenote Cristalino
In that large territory there isn't damage because of the clean gets-way escape space
In the comparative sized puddle that is your 75 gal, there is no where to escape, so any other cichlid will have to battle until death.
Below another video of the kind of available space JDs have to live in, in nature.
Eden2
In nature the GT has a similar territorial need.
I can't speak to the BP because they are not real species, but man made mutants.
But mutants from other natural cichlids with similar needs.
