Please consider switching back to breastmilk! Obviously your little girl digested it much better, and believe me, it might feel like a "sacrifice" now to breastfeed her for a bit longer, when you look back 20 yrs from now, this will be such a short span of time, and SO worth it. Did you know breastfeeding reduces YOUR risk of breast, ovarian, uterine, & endometrial cancer? That it reduces YOUR risk of type 2 diabetes? And it also reduces your baby's risk of cancer & diabetes, a decrease in overall length and duration of childhood illness which means fewer dr visits, and will save you *many hundreds* of dollars (if not more) in formula costs alone?
Please, please try the breast again - I know it's maybe not what you were hoping to hear - but you can certainly switch her back to the breast. Please consider it! If you choose to breastfeed her for even 6 months, it's such a huge benefit to BOTH of you healthwise, and it's free, always ready, and will save you $$$. If you are even remotely considering switching back and would like support of other moms who have "been there" and gone through it, check our your local LLL meeting:
http://www.lllusa.org/web/DaytonOH.html
Even if you don't give it a go again, 3 weeks of breastmilk is *wonderful* - many moms never even try to put their baby to their breast, so you've already given your daughter such a gift! So either way, you've done a great job! :)
If it helps at all, I *promise* that after 6-8 weeks, breastfeeding becomes considerably easier. If you can make it to 12 weeks, you're GOLDEN. And if you're working full time, it can still be done in many working situations -- you can pump (a good double electric pump like the Medela Pump In Style is still cheaper than formula!). I worked 50+ hrs a week and pumped, so I know it can be done. Please, I know others will say "just do the formula" and you may already be completely decided on formula, and if so please just treat my response as a "congratulations on breastfeeding your daughter for 3 weeks, and I hope you find a formula that is gentler on her tiny little infant tummy!" Kellymom.com has some good pointers on how to tell if your baby is fussy or suffering from something more (like a sensitivity to the cow or soy proteins in formula):
http://www.kellymom.com/babyconcerns/food-sensitivity.html
Congratulations on your new baby girl... they are so amazingly sweet!