A.P.
I understand what you are saying. My mother doesn't like pacifiers, but wishes she had used them on us. You can take them away, but you can't a thumb. My youngest (now 18mo) was given a pacifier since birth. He would find his thumb and seem to prefer it. Knowing how hard it was for me to stop sucking my thumb, I didn't really want him to start. I didn't worry about it when he was sleeping, but when he was awake, I would switch his thumb for pacifier. Within a month he didn't put his thumb in his mouth. He was fine with the pacifier.
I wouldn't worry too much about the 3 year old. My oldest was three when the youngest was born. He had been broken from the pacifier for over a year/year and a half. He never picked up the habit. Anytime he would try and put it in his mouth, we just reminded him that it was his brother's and he is such a big boy he doesn't need them any more. They we would have him give it to his brother to be a big helper.