Messaging app multi-plateform

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Android

server

iOS

Hi,

I have to developp an application for iOS, Android and Web application.
A user connected can send a "task" using a form with some attributes : where, when, what...
Another user will receive this message and validated the message. When validated, other users can accept a pourcentage of the task or the full task.

Do you have some suggestion to do this ?

What I have tried:

I'm thinking about Firebase but I'm opened to all suggestions