I know that all of them mean "to have to" in some form or another, but are they interchangeable?
First let's clarify the definitions of 須 and 需
須： Adverb/verb, meaning should, must; 《必須，必定，須要》
You must participate in this meeting.
需: Compound, meaning need, require; example compounds: 《需要(require)，需求(demand)，必需(need)，急需(urgent need)》
Plants require sunlight and raindrops to grow
It is very easy to mix up 必須 and 必需, even for native speakers, as they have the same pronunciation (cantonese and mandarin) and very similar meanings. In english, both can be represented by "need". "You need to participate in this meeting" and "Plants need sunlight and raindrops to grow".
The easiest way to distinguish the two is to see whether it is a noun or a verb that comes after it. If it is a noun, use 必需， 需要; if it is a verb, use 必須.
得 is quite interesting because it can be used to replace both 必須 or 必需
Wrong doing needs to be critiqued, good work needs to be complimented
It takes 3 months to finish this construction
Note that 需要 can be used as a noun as "need(s)"
As a general rule,
"須" refers to something mandatory due to regulations, rules, logics, culture, etc. It is usually used as an auxiliary verb (助動詞) or adverb.
"需" refers to something essential or definitely required. It may be followed by a verb, though the sentence will become clumsy in many cases, e.g. "植物需接受適當的陽光和雨露". It is usually used as a noun or verb.
For regulations or contracts, "應" > "須" > "需" > "得". "應" means undeniable responsibility. "得" is favorable but not mandatory.
For official documents, "須" refers to actions and "需" refers to objects. e.g. "須會商" (discussion required) vs "需會商之案件" (file requiring discussion).
These characters are very confusing, not to mention grey areas and common misuses. Have fun ~~~~
I think the short answer is
must 必须 have to 得 need 需要
First Certificate Expert Coursebook By Jan Bell and Roger Gower said:
- must is used to express strong, personal obligation or necessity:
I must post the letter straightaway. (I, personally, feel it is necessary)
- have (got) to is used when the situation or someone else (not the speaker) makes it necessary:
I have to work late tonight. My boss says so.
-need, you personally need something to do so
I need to invite some friends.(so this party will be awesome)