I would find out if the condo association requires you to pull a permit. Many of the Associations will. Plus, if you are having contractors work in the unit, the Association (and you yourself) may want to protect against potential damage by making sure that the contractor is insured and bonded.
As to whether it is legal - if you want to do it properly, you would have to call the Brookline Assessors Department to come in to reassess the unit in its new condition (3br/2ba) as opposed to a 2br/2ba. This would likely result in higher taxes. However, your unit would be stated to be a 3br/2ba at the Town Hall.
To be 100% sure, talk to an Attorney. They will be able to give you more accurate answers about the legality.... more
You should probably check with the Town's building inspector. When you apply for the building permit, he should be able to answer all those questions. If you do not get a permit, it doesn't really matter what size the door is, it won't be legal. Any future buyer will ask to see the permits and if there is none, it could pose a problem down the line.