This may just be my lack of TS knowledge, but without | null | undefined doesn't that make things like this illegal:

sourceCode.getText(null);
sourceCode.getText(null, null, null);
sourceCode.getText(undefined);

Accepting these can be useful in cases like:

const node = someCondition ? someNode : null;
const text = sourceCode.getText(node);

Currently I think we haven't enabled "strict" TS check, but I think we want to in future (and even if we don't, presumably user may enable strict option, and we want our types to support that).

sourceCode.getText(null);
sourceCode.getText(null, null, null);
sourceCode.getText(undefined);

Yes this wouldn't be allowed, but you could just pass in undefined e.g.

sourceCode.getText(undefined)
sourceCode.getText(undefined, undefined, undefined);

This change also makes it closer to eslint's exact api

I would prefer a closer solution to the eslint API.
But maybe you want to introduce a helper type to make it cleaner, something like:

type NullishAble<T> = T | null | undefined;