How and where is it better to register an enterprise for a mobile application for services! (app is free.) ??

Question

Mobile development

Hello, we are developing a mobile application for services. Mobile application - free!
Monetization at the expense of performers who fulfill orders. (pay for the received order).
I do not quite understand how best to register an enterprise, and where to register it. Many say that it is better to register in Europe.
We plan to work in Russia, Ukraine, Turkey

Answer 1

It is logical that there will be three of them, one in each country

Answer 2

This is not related to the mobile application, with what it does.
This is due to the way - how exactly you are going to accept payments