3 Months is 90 days.
I assume you choose that period because that is what pro fighters typically use to get in "fight shape".
Of course it all depends on prior skill and so forth BUT starting with a complete noob with NO prior fighting experience at all...
You can get ANYONE in fighting shape in 3 months.
To what DEGREE they are competent depends on the person.
They would have to follow at least a 5 day protocol for the bulk of it ( 6 if that can handle it), something like 3 X a week the first week, 4X the next and 5X the next.
Focus will be spent on drills and sparring with a gradual increase in intensity up to full contact at least 3 weeks before the competition ( light sparring and drills the week of).
In short, you would do specialized training with concrete and defined SHORT TERM goals.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !