To get to a mvp and prove that you're concept work, you don't have to (and maybe shouldn't) build a native app for every platform. The goal of the mvp is to get the product in front of people as fast as possible, to see if they would spend a few minutes using your products, or if it's not useful for them. Once you validate this, you can think of expanding to other platforms.

I would suggest to only go for the platform where you're most comfortable developing with. Web app can be good indeed, but if you already know objective c or java then you'll be probably better off going for native iOS or Android first. Good luck!