Unfortunately, I'd have to agree... that 3 months is waaaay too little time to learn a concerti and be able to compete (and do well, that is). 6 months-1 year is more realistic.
And this is ESPECIALLY if you are playing a complete concerto... with one movement you might be able to manage; with a complete concerto, you're probably not going to be able to. I know this from experience- I competed once with a complete concerto that I had rushed to learn, and I was playing wrong rhythms and having memory slips all over the place.
To answer your question though, I think your concerto suggestions probably aren't the best for your situation, although if you shoot for next year, I think Schumann would work very well for you (looking at your repertoire).
Your solo repertoire is pretty advanced, but Haydn D major is one of the most entry level concertos (so I presume you learned it when you were 9, 10). So you'll probably need more experience with the form and with playing with other people to learn a huge concerto. I'd recommend then, Mozart 12th or maybe Bach D minor, or Kabalevsky 3rd (if you have a choice, the 3rd movement is what everyone plays). And some more advanced ones: Beethoven 1st concerto, 2nd concerto (esp. the 3rd movement if you're playing a single movement), Mozart 17th, 19th......
TL;DR 3 months isn't enough/a lot of time to go compete; maybe shoot for next year? learn early Beethoven or middle Mozart, or Bach, or Kabalevsky 3rd (it's called the "youth" concerto)