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)

Hi! I have to go home. You can say我必须回家。我要回家（withou 去！because回is a verb means return and "去" is a verb too means "go to"! you can say 我要去学校—I have to go to school.） 必需 means necessary. Usually we use like this: 水是必需的 (so 必需 is a adj.) "I have to" you can say in these pattern: 我必须....我要....我得(dei the 3 tone).... + Verb I hope you can understand me! If not, you can send me email. I'd love to solve that for you!