What would be the difference between the three main candidates, in practice?
I think Obi of LP is more left-wing while the other candidates are from the Right. Of course, ideology does not matter. Identity does. The way the Northwest voted (Obi <1%) says exactly what they think of Igos.
Yeah, you have two main divides here: Mainstream religious denomination - Christian v Islam with syncretic and local faiths in the mix, and main ethnic group - Hausa Fulani, Kanuri, Yoruba, and the multitude of Igbo groups, as well as many smaller groups.
There are two traditional parties, APC and PDP. PDP services the Christians and southern groups more, APC the Muslims and the northern Groups. However, there usually has been a rotation in religion and regionality of the candidates between cycles so the bases don't solely produce the candidates. APC this time has a Yoruba SW Muslim as their presidential candidate with a North Muslim as VP, PDP has a Central Muslim with a Southern Chrirstian as VP. Obi is a former PDP'er but broke with them complaining of internal corruption and has coopted the LP ticket. He is a SE Igbo Christian with a Northern Muslim partner for VP.
So one way to look at it is that the unpopular APC government is turning so many people off right now that there is space for two PDP candidates. While runoff transfers may not be 1 to 1, especially if Abubakar and the PDP make the runoff in regards to SW turnout, both would probably win such a contest.