I would say for three, pretty much 150-180 gallons minimum. Not only that, but more times than not, two will pair up and beat up on the third ... and it is pretty much a certainty when you have them crowded.
There are a lot of reasons that Oscars need large tanks (or high gallons per fish) ... one is cleanliness (or rather messiness), one is the sheer size of the adult fish (about 15" long and more than 2" wide), and third is territory. Imagine being locked up in a bathroom with two other people ... but you can never leave it and it only gets clean when someone else comes to do it. That is essentially what happens when Os are overcrowded ... it becomes a claustrophobic filth pit.
Even if by some miracle they didn't fight, you would be changing water most likely 2-3 times a day at 50% or greater each time just to keep nitrates under 20 ppm. If you didn't, they would be highly prone to disease, stunting, and shortened lifespan.
I don't mean to sound harsh, but I want to be straight-foward about the risks, issues, and reasons. Oscars are wonderful fish. I would love for you to be able to keep them all (I know how easy it is to get attached to them) ... but unless you can buy a large enough tank to put two of them into, I honestly think it is best for the fishs' health, and your sanity/emotional well-being to consider taking a couple of them back.
I will tell you, one Oscar in a 55 gallon can be a lot of work. Mine was growing over an inch a month, and eating tons of food. This creates a lot of waste. I couldn't imagine three ... I was changing the water several times a week (large amounts to boot). Now that he has stopped growing (hit about 10" with tail), his appetite has slowed down and I can stretch the changes a bit further.
It is always best to err on the side of caution. I personally feel that the 50 per Oscar is too small, and it really should be 75 each. So, 4 in a 300 gallon ... but I think if they were friendly, you might squeeze 6.
Same with filtration, I would rather be a bit over than know I am under. I have 750 gph of (hang on back) power filters on the 55. I still don't think it is enough, and will never buy a power filter again, from now on, canisters.
Oscars if taken care of properly and healthy, do grow an inch a month give or take a quarter to a half inch. The first month I had the brat, grew 1.5".
Remember one very important fact about nearly any 'store' ... it exists to sell and make money, and not everyone thinks making money involves informed employees giving their customers informed decisions. If you have studied a bit on proper fish care, and walked in poorly run store, you will pull your hair out just looking around, and have to take deep breaths not to be obnoxious when a employee rambles about nonsense you KNOW is incorrect. Finding places like this, where people love and care ... you will always find better information that suits both the owner and the family member (pet). Think about it, if a store told you a cat (or dog) could live in a cage its whole life, they wouldn't be lying, but, the cat (dog) would not be happy, and most likely not healthy.
Take care.
