Stories Poker Sizing - How do we estimate mobile apps effort?

Hello,

If we are doing stories sizing for a product that has one backend, and multiple front ends (Web, Androind mobile app, iOS mobile app), what is the best approach to do the poker sizing with?

Splitting the stories like: 1. Add new User on iOS app, 2. Add new User on Android App, 3. Add new User on Web. The idea is shall we split the same backend effort on the 3 stories or what?

Thanks.