Well if it's any consolation, my ds who just turned 4 was 3.6 before he was dry during the day and only got the hang of doing poos in the toilet 2 months ago. He has just decided to finish with night nappies, so we just got the potty training completed by the time he was 4. No developmental reasons why he was slow, he was just very anxious about the whole process (even though he has a big sister who has been through it all and was fully trained by the time she was 3!). Honestly, it's probably best not to stress too much about it' which is easier said than done I know!
My youngest is 3 in nov and is not remotely interested in potty training. He understands what the toilet is for and knows when he needs to go but refuses to go other than in nappy or pull up. He sits on the toilet happily swinging his legs and telling us he won't wee in toilet. Then 2 mins after taking him off the toilet he's soaking. Any suggestions welcome