no, in a word, he isnt.
jacks, though slow grows compared to many other cichlids, grow to 8 inches and are quite aggressive upon maturation......there is no way 3 adults will live peacefully in a 55 gallon....no way.
for a while will they? maybe, probably, who knows.
my advice if you like jacks?
raise out about 5 of them until you get a breeding pair, then move the others out with great haste!!
(if you have one that is getting picked on heavily, pull him/her out first obviously, you dont want dead fish)
a single oscar would be a nice "wet-pet" for your tank and they grow MUCH faster than jacks.
it is up to you.