Look up the spoiler effect. Please! This vassal is begging you.
The question under your system (please inform yourself about first-past-the-post) isn't who do you want to win, it's who you do you want NOT to win.
If you vote for your third-party candidate, it's equivalent to not having voted at all, if they have no chance of winning.
You're going to get Biden or Trump with how people vote (spoiler effect, look it up), one of those is going to win, make your peace with that.
So, which would you rather?
I am happy to spell out in greater detail why voting for a third party candidate is a waste of time under your system, happy to chat if there's still any confusion about it.