I tend to agree with AnnieBrooke, it may be an attention thing. She may be jelous of the baby & feels this is how she can get more attention. Try having some kind of fun time just you & her. Ask her what she wants to do for a whole day & don't mention potty training at all, just change her when she needs it. I know when my little one doesn't get enough attention from either me or her daddy she finds ways to act out, & it sounds like she is doing exactly that. If you need bigger pull ups, I know alot of stores don't carry the bigger sizes, try Diapers.com they ship it to you for free, it only takes 2days & they have the best prices around. I wish you the best of luck, let us know how it goes.
My son will be 3 in January and has almost no interest in potty training. He will sit on it, but still not comfortable going in it. We have just completely and totally backed off and are allowing him to go at his own pace. I have spoken to his pediatrician, and she agrees that this is the best way to work on it. He knows where it is and what it is for.
I agree with all that AnnieBrooke says. When kids are pushed, especially toddlers, they will push back. She might be afraid of it (my son worries about falling in) or may just be reacting to feeling pushed.
I would just let it go. She'll get there. And at 3 it is certainly not abnormal to still be in diapers.
It is quite possible to get kids' diapers in larger sizes than would fit a 4-year-old. My nephew is still sleeping in a night diaper, and he is 6 and a pretty tall kid.
I react to your post by thinking that you sound like you are pushing your daughter on this a lot, because you want to save money and because you are tired of changing diapers and want her to change so you won't have to attend to this. And she is saying by her actions, "Don't push me," and maybe even "Did you notice that I have a 7-month-old rival for your attentions?" Does she get your attention in general, or is this the easiest way for her to get it on a busy day full of work and baby? Maybe it would be a profitable line of experimentation, to give her some attention when it's not about potty training and diaper-related moments, and see where that goes. Right now, she might get your undivided attention at other times, but it sounds like the one time she is CERTAIN of your undivided attention is when she is being changed.