First of all congratulations for breastfeeding. ITs wonderful taht you have given your daughter 3 months of the best start in life!
Now are you pumping and bottle feeding, or nursing and only pumping when you need a bottle for her? And you are going back to work soon? What kind of pump do you have?
Going back to wokr and breastfeeding will work just fine! You need a good pump though, and if you don't already have one, invest in a double electric pump, like the Medela Pump in Style or Pump in Style Advance. Your insurance may cover a pump since yo uare returning to work, so check that out too. Pump 2-3 times a day at work, and nurse her when you are together and on the weekends full time. Nursing on the weeknds full time will ensure that your milk supply will be full force come Monday for pumping.
IF you choose a formula, go with the most basic you can find, and introduce if SLOWLY. You can combo-feed, whic means you'll use formula at daycare and nurse when you are together. Your daughter will still get the excellent benefits of all your breastmilk, the antibodies and the brain building nutrients if you nurse a few times a day!
There is a solution in there tha tworks for you, whether its investing in a great pump to keep up with your daughters supply or combo feeding, using formula at daycare nad nursing when you are together. Both are great options that will allow your daugther, and you, to continue receiving the benefits of breastmilk!