Oooh I have good news to share with you ladies! My 3 year old DS was doing this too, for months and months, and it was driving me insane, I was worrying he'd never poo in a toilet and would be in pull ups til he was 7 or something.
On the advice of a child psychologist we took all pressure off, but the big change was that when he did a poo in his pull ups we'd take it to the loo and tip the poo in "because that's where poo goes, isn't it?", so it did a big 'plop' into the water (talked about how funny it was, etc) and then waved 'bye bye poo' and let him flush it away.
We also talked about what poo is - i.e. the remains of food he's eaten that his body doesn't need, so we'd talk about him having a spaghetti poo, or an ice cream poo or whatever, this seemed to help him be less poo-phobic. Also talked about 'when you're a bit bigger you'll poo on the toilet' and didn't make a big deal out of when that woudl be, just let him know it would happen, when he was ready. Also made sure he ate loads of fruit and drank plenty of water / juice and used lactulose every other day or so, to ensure poos were soft, so it didn't hurt to go.
Literally a week later, he suddenly decided to go and do a poo on the toilet, all on his own!! I've never been so utterly gobsmacked in my life!! Sometimes they just have to be ready to do it. He's done it 3 or 4 times since, and is chuffed to bits with himself.
Keep telling your DC gently and encouragingly that whenver they're ready to do it, that's fine, and make sure they're happy about what poo is and where it goes.
I know it seems like it'll never get better, and I really was seriously worried that my DS would never sit on a toilet let alone poo on one, but lo and behold, eventually when they're confident they can do it, and that it's not scary, they will.