My daughter had the same problems when she was young, she never hold it in tho she would just sit on the toilet and scream and cry trying to get it out. She's 9 now and still has poo so big I can't imagine she gets it out (she never flushes the toilet so I get a surprise after she goes, lol).
We tried a concoction of apple, cranberry and prune juice, it works wonders, and also a bowl of raisin bran in the mornings or as a snack.
I can't believe I am sharing this......lol
I recalled all of this info when I went thru the same issues with myself in the last few weeks. I'm pregnant and it seems my body does not agree with the pregnancy. I just couldn't go...well, I couldn't pass the stool. I tried everything, the docs didn't think it was that big a deal....they didn't see my belly!
I'm 4 months pregnant and my belly was so distended I looked like I was 9 months along and you could feel the impacted stool (solid as a rock) in my abdomen!!
It was when I had to start "removing" the stools myself(NOT pleasant at all!) that I had had enough!
I got the juices, mixed them, ate the cereal and had to result to an enema. After 2 days of that, I was cleared out and my belly shrank back down to it's normal size.
The laxatives for me were no help, it was the natural course that got things back to normal.
Maybe try this method and use the pull ups, maybe if you can get her regular again, she will stop holding it and use the potty again.
There is nothing more scary, painful and stressing than when your body doesn't work things out like it should....especially for a 3 year old that doesn't understand what is going on.
Best of luck to you and I really hope things work themselves out.