You don't say if your daughter is in pull-ups/diapers, but I'm assuming so. I don't know if you've tried this method, but it worked for my (nearly) 3yr old daughter. Set aside a few days when you have no big plans and don't have to go anywhere. Then, tell her that she's a big girl now and knows how to use the potty, so she's not going to wear pull-ups anymore. Then, either put her in underwear, or since it's warm now, let her go naked from the waist down. With this method, I didn't even need to do what some others suggested and have my daughter sit for long periods on the potty. All I did was put her in underwear and reminded her that she was NOT wearing a pull-up, so to "run to the potty if she felt pee or poo coming". She had one (wet) accident, which I did NOT scold her for, but DID have her help me clean. I only said "I know that since you're a big girl, next time you'll try your best to go in the potty." After that I just didn't put anything on her bottom, and let her run around naked - all day. When it came time for poo, she nearly freaked out when she realized it was coming and there was NO diaper/pull-up to go in. There was much crying on her part, but all I did was remain calm and remind her that if she felt poo coming to go sit on the potty. I did NOT force her to go, or even bring her to the potty (but I did have a little potty in the play area, and one close to wherever we were throughout the day). I was fully prepared for another accident, but guess what?? ... she chose to go on the potty, rather than poop all over herself or the floor! I know people think this method is odd, but my sister potty trained her 3 kids this way, and all were trained within a few days. It only took my daughter ONE day, after many months of trying with the pull-ups. I understand now that the pull-ups were just giving her another way of peeing & pooing the only way she ever knew - in her diaper. Putting pull-ups on kids just delays the whole process in my opinion. I believe it really took getting rid of them for my daughter to "get it" about how her own body worked. Of course we used positive reinforcement - after each success on the potty, she got a few m&m's or a special wrapped (small) gift for poo. We also read potty books on a daily basis. We do use pull-ups at naps and bedtime until she can remain dry while sleeping.
Good Luck! I hope this helps! I know how it felt to be at my wits end when nothing else seemed to be working. I really thought this method was crazy....until I tried it. Be consistent, and once you have her in underwear, don't go back!