Neither 1 nor 2 is natural. You'd say:
I'm running a 38.6 degree fever.
I'm running a fever of 38.6 degrees.
3 is fine. It's normal to use the present simple (is) because this is an undisputed fact - a date on a calendar. There is no need to use the 'going to' future, because it's not an opinion, plan,intention or prediction.
By the way, 'gonna' is not a tense. You shouldn't write this unless you are transcribing deliberately informal speech, for example in a subtitle, cartoon caption or song lyric - or possibly in a brief text. Please don't make the mistake of assuming that this is everyday written English.