いずれにしても / いずれにしろ / いずれにせよ

いずれにしても, いずれにしろ and いずれにせよ mean "in any case" or "either way": whichever way the open question turns out, what follows still holds. They stand at the start of a sentence or clause, usually after you have laid out the options or admitted what you don't know.

いずれ here is an old word for "which (of them)". On its own, いずれ more often means "some day", as in いずれわかる分かる, "you'll understand one day".

Which form to use

The meaning is the same; the register is not. いずれにしても is the neutral form and fits both speech and writing. いずれにしろ uses the imperative しろ and sounds brisker, more spoken. いずれにせよ uses the literary imperative せよ and belongs to writing, speeches and news commentary.

Everyday alternatives

In conversation, どっちにしても or どちらにしても say the same thing when there are two options. とにかく ("anyway") is more abrupt: it brushes the details aside and moves on, without the sense of having weighed the options.