Here is a handy little calculator from kellymom that will help you figure out how much LO likely needs at each feeding. BF changes consistency (calorie content, etc) as your baby gets older so he will likely take the same quantity of BM from now until he is about 6 months old with a little extra thrown in for growth spurts. If he is consuming a huge amount (say 35-40OZ a day instead of 20-25OZ a day) he may be needing to comfort suck a little in between feedings and is getting milk when he does that or the bottle you are using may be too fast for him and he isn't realizing that he is full until he has taken in too much milk. There are several helpful tips on that page to help you determine if he is getting too much/too little milk. HTH and GL!
Re: how much BM for 3 month old
Mine takes about 3.5 oz. every 3ish hours.
Sometimes it's 2.5 and sometimes (like before bed) she'll down 3 bottles in a row. She's such a random kid.