It sounds as though your son’s refusal to sit on the potty is part of a general pattern of immaturity. Also, it is possible that you tried to start him too soon and built up resistance. I base that on your statement that you’ve only been able to get him to sit on the potty “three times in the last three years” and he is not yet 5.
A first step is to make certain you know his pattern—when he is likely to have a bowel movement, how long he can go without having to urinate. Write down your observations; don’t trust your memory. Then try the potty when you are pretty certain something is likely to happen. For his urine, I wouldn’t hesitate to let him go in a somewhat private part of your yard (if you have one). Little boys often like that privilege, and it’s a lot easier to manage an arrangement for urination without the ordinary potty equipment for boys than it is for little girls.
Finally, I would try to use his sister and brother as allies. Ask his sister to stay with him in the bathroom and perhaps share a book with him while he sits there. Try to keep him in the bathroom with you when his little brother uses the potty. After you praise the little one, casually ask, “Do you need to go now?” I know it’s hard, but try not to shame him, and do your best to keep your 7-year-old from doing so.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.