i think 必须:when you think one thing that you must to do,eg 我必须回家 that is say nothing can Impediment you to go home.必需:just like a soap,if you haven,t it,you cannot take a shower,香皂是洗澡的必需用品。要:just like "want to" it cannot see that it would like to do or not,so it's a neutral vocabulary.eg:i want to go home(that is you could stay here for a minute)