Introducing a new and young rat to my other two rats?

it would be a REALY bad idea to put the rat in there and just leave them be.. you should take them out and introduce them and shw them they need to be 'friends'. but with the rats age.. im not that sure, we had a rat once and she died last year but never got the chance to get her a friend, but ive helped my friends introduce animals together and most turned out fine unless they moved to fast or the older animal didnt trust them enough. but you would need to ask the store/center that you et the at from how big it would have to be.