算了 / 行了 / 得了【suàn le / xíng le / dé le】

算了 drops a plan, a request or an argument: it isn't worth it, so let it go. It stands on its own or opens the next sentence.

…就算了

A clause + 就算了 lets one thing go: "if X, never mind". Said about someone else's choice, it can sound sulky.

行了 and 得了

行了 and 得了 tell someone to stop: "that's enough". 得了吧 goes further and rejects what was said, "oh, come off it".

行了 also has a neutral sense, "that does it": 这样就行了 means "that'll do".

Same characters, different words

算 + 了 + an amount is the verb "work out" with a completion 了, not "forget it". And 得了 here is 得了dé le; after another verb, 得了 is 得了de liǎo, "able to", as in 去得了.