or just let him do it in his own time? He is only just 3, my lad was still in nappies at that time but we were trying pullups with him, the ones where the picture on front disappears when wet.
he may also be uncomfortable on a potty, try a seat on the toilet, or get a family seat like this
http://www.heatandplu...se&utm_campaign=Feed1
when we get warm weather try leaving him with no nappy at home.
also try and get his sister to start trying to use potty / toilet too, girls are often a bit easier than boys to train.
the more pressure thats put on him, the less likely he will be to be at ease with it all and it will cause stress for all