Do you know for SURE that it is "gas" problems?
Your baby is only 3-weeks old, and it could be a combination of gas and/or digestive problems or something else.
For the gas/digestive problems: try (1) "Hyland's Colic Tabs"...it's for all ages, and helps for ANY gas/digestive problems in babies. It is NOT only for "colic" babies. Our daughter had bad gas problems too as an infant and this helped her a lot.
(2) As the other poster said, it could actually be your breastmilk and what you are eating, that is giving her gas. And not the Formula per say.
(3) Sometimes, a baby/newborn is fussy because they are actually not getting enough intake. Is your baby latching on fine? Is she actually swallowing and can you hear it? Is she having enough wet/poopy diapers? I am asking this because I have seen this many times...
(4) Do you burp baby often enough?
(5) What kind of Formula were they giving her in the hospital? Did this formula cause her problems? Or just the Similac? I would really ask your Pediatrician for recommendations... and about the gas as well.
(6) a baby full of "gas" and gas pains... many times can't exactly "eat/nurse" very well. It's uncomfortable and their tummy is bloated. So, make sure she is getting enough intake...otherwise you will be dealing with an infant that has gas problems AND is hungry all the time= fussy and inconsolable baby.
(7) You mentioned baby had breastfeeding problems in the hospital and thus, you/they started supplementing while working on it. NOW... what is the status of this? Is she now nursing fine? Or not? Is she still "needing" to have supplementation by Formula or not? If not, then just nurse her... but only if she can now latch on properly. And then you won't have to deal with "which" Formula to give her.
(8) If she is indeed having gas.... try gently massaging her tummy in circular motions, or bicycle her legs... and this helps to "circulate" the gas and get it expelled.
(9) ALSO though... your baby is only 3-weeks old....MANY newborns have gas/digestive problems because their digestive system is simply STILL developing... and its a given that their system is still getting used to everything. As a baby gets older... it usually gets better. But so it takes patience.
(10) or, some babies simply swallow a lot of air while they are drinking, thus, gas. For me, I LOVE the "MAM" brand baby bottles... which you can get from www.amazon.com My Son loved this brand/bottle, and it contains NO "BPA's" in the plastic. ALSO... while the baby is drinking, there is ABSOLUTELY NO AIR BUBBLES which accumulates in the bottle. I really recommend this bottle.
Well, just some ideas... all the best,