Several things...
As youre finding out, FHs usually want a tank to themselves. If youve tried 1 tankmate, youve basically tried them all with these fish. Ive never had luck with adding tank mates with one once it hit about 4 inches.
3 cons are just 3 things to kill one at a time instead of 1 at once.
Now, I wouldnt recommend this unless youre willing to give it the attention it needs, but it is possible that a large spawning pair of cons would work. If it were me and I really wanted tank mates, thats what Id try.
Id divide the tank, let the cons spawn then remove the divider so the FH got the brunt of the cons willingness to destroy him and see where it goes from there...
Ive had a pair of cons take over 80% of a 125g with a large RD, large female festae and large freddi all huddling in the top in fear of the con pair, as well as having a con pair do the same in a 265 with more large fish. In both instances, I removed the cons.
Pound for pound, a spawning con pair is as tenacious as the largest cichlids alive.