At 3 months I would be expecting about 6oz per bottle really. Some babies take less, some more. A lot of formula fed babies are needing longer than 3 hours between feeds at this stage too.
Have you tried a faster teat? You can encourage them to go longer between feeds by stretching the time. So, start with the first feed and don't feed her until as close to 3 hours as you can get without her getting too distressed, so distract do what ever you can. Then feed and because she is that bit hungrier she should take more (using a faster teat can really help providing it's not too fast for her). The next feed you make sure it's at least the same time between the first two feeds, or a 5-10 mins longer if you can and you keep going like that until you get to the 3 hours. It will take a few days to gradually stretch her.
Laura