Example:
I wasn't sure if it was A, B, or C. [...], I still knew I would fail.
I thought of either way but I think is not grammatically correct.
What should I chose if I'm referring to two or more options?
How about "whichever" or "no matter which...?"
I wasn't sure if it was A, B, or C. Whichever (or no matter which) way, I still knew I would fail.
Either is not advisable in written English, though you hear it often enough in spoken English. My choice would be, "Whichever, I still knew I would fail."