All 3 of my kids have juggled both, usually earlier than 3 months. With my 4 month old I started him around 10 weeks when I had to go back to work, I would pump breastmilk each day but it wasn't enough so he started taking a little formula too. He is 4 months old now, primarily drinking formula but I still nurse him a few times a day and in the middle of the night. I've just never been much of a milk producer.
I would first make sure she can take a bottle of breastmilk. Can you pump some (if you haven't) and get her used to drinking from a bottle? I think she is probably at a good age to try a bottle, if you wait too long, they will refuse (like 7-9 months I think)
None of my 3 kids ever had issues juggling formula and breastmilk, once they were drinking from a bottle anyway. It DOES turn them away from breastfeeding a bit though. The bottle just comes out so much faster and easier for them. Sometimes my 4 mo old seems too impatient for the breast and it takes awhile to calm him down into it. But at night and earlier morning he prefers to breastfeed. Your supply then begins to dwindle as well, if she gets a bottles of formula every day. So it depends where you are with the breastfeeding and if you are okay with that. Personally, I am fine with just breastfeeding a few times a day now for the comfort aspect and to keep something flowing. Mostly, I don't want to get up in the middle of the night to make a bottle so it's nice to keep the option of breastmilk :)
Sorry if this post seemed to be "all about me", but I can only speak from my own experience, I don't know what will happen with your little one and how much longer you plan to breastfeed. But it is possible to do a combo of both, if you want.