算了：有give up的意思在里面。主要指自己放弃。Eg:there is a thing,both of my friend and I want it.So decided to give it to him.Then I can say：算了，我不要了。 行了：enough,Ok.对自己和对别人都可以用。行了，你可以休息了。/行了，我要回家了。 得了： to stop it， to hold it ，主要是对别人说的。Eg：你得了吧，就你这样还想当老师！

算了：不再计较，罢了 例：算了吧。我刚才太激动了。我道歉。Let's forget it. I was too excited just now. I apologize 没人会吃这种食物，还是扔掉算了（罢了）。No one will eat this food; it might just as well be thrown away. 行了：好了，就这样吧 例：行了，行了，我知道。不用这么大声。Yes, yes, I know. No need to sound so loud! 我想自己可以，只要知道方法就行了。I think I could,if I only know how to begin 得了：（1）用在反问或否定句中,表示情况很严重 例：这还得了吗？ （2）得了吧，我不去了。（算了吧的意思） （3）他得了第一名。He won the first place 他得了这种病。He suffered from this disease

when you are disappointed.you usually say 算了.when you want something over.you usually say 行了.when you have made something finished.you usually say 得了.but,if you don't agree with others opinion. you may say 的了吧.