All of these words mean must or inevitable, but is there a difference between them?
- 不得不: have to, there are factors/reasons that force someone to (do something)
I have to turn off the machine due to overheating.
I have no choice but to turn off the machine due to overheating.
- 不免/难免: would naturally; cannot help but.1 (It sounds more like reasonable than inevitable. See example below)
It'd be normal to make mistakes due to the high workload.
(The mistakes can be avoided if careful enough. But due to the workload it is very reasonable to make mistakes. You can use 必然 here)
- 必然: inevitable, certain. (At least the speaker believes so. There may be reasons, or there may be none)
There must be other forms of life in the universe.
(The speaker believes so, but he doesn't necessarily prove it. There may be no reason given for the statement)