My 3-year-old is quite happy to sit on potty or toilet but won't go. He will go in his training pants and hides when doing a poo. How do we get him to go?
Potty training is all about rewards, not punishment or pressure. Your child needs to want to use the potty, so you need to think about what will motivate him to do it. The reward needs to be immediate, not long term. I've found TV or using apps on a tablet to be the best motivators, when my children did a pee in the potty they got to watch a short program like Octonauts, Dora the Explorer, and the like. If they did a poo they got a movie or a longer TV program.
Once that was working I would gradually move the goalposts. At first if any pee went into the potty they got TV, eventually it was only if they did a pee in the potty with none in their underpants did they get TV.
Another great reward is a star chart, a pee gets a silver star while a poo gets a gold. Many children do really well with star charts.
Of course praise is the best motivator of all. The first time he does a pee in the potty make a really big deal out of it. Tell him what a big boy he is, call his grandparents and tell them about it, get them to tell them what a big boy he is and how proud they are of him. Let him pour the contents into the toilet and flush it, because that's what big boys do, that sort of thing.
Whatever happens be patient, and don't get angry when he doesn't do it. You want everything around the potty to be positive, and non-pressured. Don't get in a battle of wills, and don't lose your cool. It's easier said than done I know!