In TypeScript, you would do something like

async function getString(word: string): Promise<string> {
   return word;
}

How can I do the same in Python? I tried the following:

async def get_string(word: str) -> Coroutine[str]:
    return word

And got this traceback:

TypeError: Too few parameters for typing.Coroutine; actual 1, expected 3

So Coroutine expects 3 types. But why? And what should they be in this case?

This is also specified in the docs, but I still don't understand

The example in the docs shows what the three types are:

from typing import List, Coroutine
c = None # type: Coroutine[List[str], str, int]
...
x = c.send('hi') # type: List[str]
async def bar() -> None:
    x = await c # type: int

1. what you'd get back if you sent a value;
2. what value you can send; and
3. what you'd get it you awaited it.

It also links to the Generator definition with more examples, and a slightly clearer definition:

Generator[YieldType, SendType, ReturnType]

In your case I'd guess [None, None, str], as you only care about the awaitable value.