First you'll want to quarantine the two new ones for at least 90 days and get fecals done.
Second to introduce new Leos to one that has been housed alone for so long you'll want to gut the tank clean everything and rearrange it so it's no longer familiar to the original Leo.
Three can be housed in a 20 long with no problems, but you want to make sure that they get along, and be prepared to house them separate if they need it.
I would not house the male if it ends up being a male with the females unless you are prepared for babies. If not, don't let them breed. It's not fair and takes too much out on a female to allow her to breed and not incubate the eggs. You'll also want to make sure that both females and male are up to par for breeding.
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9.23.22 Leos
0.0.1 AFT
1.1 Red Ear Sliders
0.0.1 Russian Tort
1.0 Pembroke Corgi
0.2 Cats
8 fish