It all depends really. Ideally you will always go against the highest rank boxer you can so you climb the rankings quicker. But I will normally not match my boxer up against someone much better than him. So really just the highest ranked boxer I think he will beat.
It’s likely an undisputed boxer dropped a title and your boxer became champ as he was no 1 in the rankings (or the highest ranked without a fight planned). The 3 titles have different eligibility for challenging for the belt. If no one is meeting all 3, then they will vacate a belt. You will probably need to do this later in the game as well.