With our daughter we planned a day when big 3 year old girls didn't use diapers anymore (except at night time). We also talked about when this day was coming and we watched Elmo potty time and read books about going in the potty. When the day came we had a special celebration of what a big girl she was and just told her there were no more diapers to go poop in. So you could try that... if she says she knows there are diapers because she uses the night time one you can say that those are not for daytime. It's interesting what kids will accept when it's said in a matter of fact way... If you choose this though, you have to be ready to follow through. If you go back on the diapers it will take longer.
We made the mistake of using pull-ups while our daughter was potty training. As soon as we figured out those didn't work and we put her in panties she was trained. There's something about not liking to be poopy in panties. If she's holding it and refusing to go I would make sure that you alter her diet to include more fiber and water so she doesn't get constipated.
As someone else mentioned I think, you may want to get a potty chair if you don't have one. Talk to her about using it before she has to go and tell her that when she has to go poop she can use the little potty. When she asks to have a diaper, don't say anything about the potty, just say ok let's go poopie and take her to the little potty and see if she'll go there?
Good luck~ she'll get it. Just try to be positive with her.