I started to train my son not to wear diaper when he is 3 years old. During day time, I let him wear panty. Haha, is so funny to see him wear his panty for the first time.
Lat time, he use to pee on his panty and need to change all the time. He definitely will feel uncomfortable after pee on pants. Then I started to train him by asking him : "Do you want go to toilet ? Tell me when you feel you want to pee or poo". Keep on ask him and let him aware of that. It's quite fussy, right ? My baby sitter will do the same also.
At home, after meal or milk - normally after 1 hour, sure I will ask him: "toilet ?" ... sometime he will said "yes", then I quickly bring him to toilet and let him sit on the potty. Sometime he told me he want to go to toilet before I asked him. hehehe ...
Night time, I use to let him pee before going to sleep. I will told him like that - "now you are not wearing diaper, if you want go to toilet, call me or papa, ok ?". After that, he will go to sleep. For the frst few days, we will wake him up and bring him to toilet, and he continue back to sleep after toilet. Then day after day, he will wake me up if he want go to toilet. So far, he has did a good job, and I really save a lot on his diapers. Only sometime may be he too tired, he will pee on his pants. Time to change his pants, and still not wearing diaper .... hahaha.
Praise to Lord, now he manage to sleep throughout the night without wearing diapers. And if he really want to go to toilet, he will wake me up ask for accompany to toilet. Normally he only wake up 1 time during midnight then go back to sleep again till sun rise.
This is the way I train my son not to depend on diaper, is quite tired but it's worth. No need to pressure the kid, in my opinion. Some kid may be able to start earlier but I think 3 years old still consider ok .... ^_^