Hi. Just wanted to say a big DON'T WORRY! My DD is 5 and half months now and we were in exactly the same position at about 3 months. She had taken a few bottles when a few weeks old and there was no problem. So despite dire warnings from my sister that we needed to give a bottle every day we didn't bother (what an eejit!).
When we tried her again at about 3-3 1/2 months she absolutely refused a bottle. It got to the point where she wouldn't have one near her mouth and would start screaming when she even saw one. We tried every teat under the sun, all the different times of day, when she was hungry / not hungry, me going out/staying in and nothing worked. So then we spent a horrible 24hrs trying the other suggested approach - don't breastfeed and just give them a bottle - they'll drink when they're hungry. Of course she didn't! DD didn't take anything for 17hours and then about 3 ounces an ounce at a time protesting all the way. After 24hrs of this DH and I were worn ragged by her heart rending sobs and were paranoid about dehydration so we gave in and it was back to breastfeeding. I really would not reccomend this approach to anyone unless you are absolutely desperate. My LO has inherited stubborn genes from both her parents though .
Anyway, a very bright friend of mine suggested giving her a bottle to play with as they start putting things in their mouths themselves around four months. We did this, and although she ignored it most of the time it did get the odd chew. At around 4 1/2 months we noticed that her taste buds had really woken up and she was interested in different flavours - the bath water, apple juice on my fingers, dad's sweaty arm (ugh). Anyway one day when she was both cheerful and a bit peckish DH snuck up on her with a bottle and after giving the teat one chew, she wolfed five ounces without a break! We tried for next few days with varying degrees of success but she mostly wanted to just gnaw the teat (Avent). So we switched to a Mam teat, and she just looked at DH as if to say 'why didn't you give me this one before daddy?'. We had of course - but she screamed at it last time. She's now on two bottles a day a nursery with no problem.
Wow, what a long post. Sorry for rambling, but it's kinda cathartic for me as it was so depressing and worrying at the time. I still can't quite believe she is taking the bottles so happily. Good luck, and I am sure your LO's will get the hang of it.