When you moved in was it stipulated that there was a pet deposit for EACH pet or was the deposit the same for each person who owned a pet or pets?
If it was not stipulated that the pet deposit was ($50) PER pet then you will be responsible for an equal amount of the damage bill.
What about the 3rd person - did they have a pet as well?
An arbitrator would have a hard time determining what pet did what damage to the suite - the only logical way to divide up the bill is split it three ways - then if you each want to quibble with each other about who owes more or less you'd have to do that in civil court, as far as I know.
Lesson for the future? Get everything, every detail about the rental rules/deposits/shares etc down on paper on move in, then there is no room for argument on move out.
Suzan J

The post was confusing. I gather there were three of you who each paid one third of the SD. Two of you owned pets and both of you each paid out an extra pet deposit.

Now, the owner is sending a bill for the damages not covered by the pet deposit and the security deposit. So the owner has kept the pet deposit that two of you paid, since you had pets, and kept the security deposit that you each paid one third of. The owner is now trying to collect more money to cover perceived damages.

Where I live a landlord must give an itemized list/receipt of all the damage and the cost of each. Did you receive such a thing? Is all the damage due to pets. Or is it also things done by the humans, like cigarette burns? Is the pet damage just one big thing like having to get new carpeting, or is it a bunch of little things like windowsills chewed, screens clawed, hair not cleaned up, urine odor?

No matter what, this is not going to end well, and I think everyone involved, meaning the landlord and each roommate, is going to feel they did not get a fair shake at the outcome.

If you can get a list of damages Maybe, just maybe, each roommate will chip in for what they are responsible for. Did "Bill" break the window??? Did "Bob" set fire in the oven?? Was it "Mike's" cat that peed all over? Was it "Bill's" girlfriend who smoked the cigarettes? Does no one remember who it was who let the bathtub overflow?

Where I live a landlord can collect for damages *in full* from ANY single one of the roommates that signed the lease. In other words, he could come after you for the full amount. It may be different where you are. If you end up with a judgment for the entire amount owed, you then have to go after your roommates in civil court.

What you ask in your post is not a legal question, but rather a moral one. I think how it is divvied up would make a difference on what the damage is. If it was all pet damage then the two roommates with the pets should pay the entire amount. If the damage is something like a broken plate glass window because of a rowdy party, then the pet owners should not have to lose the pet deposit to pay for it on top of paying the balance and pay an inequitable amount. If the damage is a combination of pet and human mischief find out the total bill and divide by three.