I don't know how much to feed my 3 month old

Offer her 8oz every four hours? That would be 32oz. I think the general rule of thumb is around 2 - 2.5oz per lb of body weight, which would be about right.

I think at that age though, you should just feed her if she’s hungry. Draining a bottle is an indication that they would take more, so I would up it by an oz each bottle.

My baby only had 4 bottles a day at that age (every 4 hrs) plus a small night feed which she dropped at 5 months.

Honestly I never track it. My baby eats what he wants and stops when he's full. When he routinely starts finishing bottles I added more per feed.

As long as your babies weight gain is fine and your baby is otherwise healthy, I think this approach is ok

It’s recommended that you pace feed and feed on demand. For bottle feeding that means offering a milk when they want it and they should take as much milk as they want so there should always be milk left in the bottle because they have decided they have enough.
If she is draining the bottle you need to be making more milk.

Bigger bottles but fewer is the obvious answer. Work out how much baby should be having in a 24hr period, and split it into 3 or 4 bottles

Don’t worry about input, focus on output. Is she weeing/pooing enough and is she following her weight curve in her red book?

Babies arent textbooks though. I doubt you eat exactly the right amount of calories per day. As pp said, worry about output rather than input. One dirty nappy a day and loads of wet one is great. If shes not waking for a feed, I wouldnt wake her. Babies will never let themselves starve....

OP i thought you said you had been weighing her? Just fill the growth curve chart out each time you weigh her

Also something to think about is that if you were breast feeding you'd have no idea how much she was taking

It may not be super accurate, but if you're using the same method each time you'll notice a trend in weight gain

Even if you are bottle feeding current advice is that it's on demand (within reason). If your baby finishes bottles they need to have more in

OP thats fine as a way of weighing them if you're being consistent, weight may be out a little but the weight gain will show

Give her as much as she wants at each feed. If you were breastfeeding, you wouldn’t know how much she was taking. Babies obey their own satiety cues pretty well, so It’s generally best if there’s always a bit left in the bottle after the feed as it shows they have fed all they want to.