I definitely would NOT recommend using the 3% internally. If I recall correctly, it contains plenty of other chemicals used to stabilize the solution, and is definitely not for internal use. There is a "food grade" hydrogen peroxide, at 35% concentration.
However, please PLEASE DILUTE THIS! If you drink that stuff straight, it will cause internal bleeding, fainting, and, depending on the amount you digest, possibly death. I really really can't recommend this, but if you want to try it, I found a guide on how much to dilute your solution.
I'd read the whole thing, but the instructions are about halfway down the page. You can make a 3.5% concentration solution out of the food grade 35% hydrogen peroxide, without any of the nasty chemicals in the stuff you found in the pharmacy.
All things considered, I'd look for other solutions, but it's your body I suppose. Personally, I found my solution about two weeks ago: apparently some sort of bacteria/infection on my tongue was causing my body odor, so after purchasing a tongue cleaner (they're by the dental floss) and using it every time I brush my teeth, I've been completely symptom-free for the whole two weeks. I've got a thread further down the page if you're interested in further information on that. If you notice an excess buildup of "tongue gunk", I'd at least consider it. Worst case scenario, maybe it'll help with your bad breath.
Hope this helps. :)
PS, in case you skimmed this, DO NOT DRINK 35% HYDROGEN PEROXIDE WITHOUT DILUTING IT. NOT ONE DROP. :P
Standard disclaimer here: I am not a doctor, and cannot be held responsible for anything that happens to you if you attempt this procedure.