backup and restore app for databases
I'm looking for an open source app to backup sql server / mariadb / postgres.

One app?  Singular?
You won't find one.
Every DBMS does things ever so slightly differently from all the others.  Most significantly, each and every one has its own utilities for backing up and restoring its databases.
There is no &quot;one size fits all&quot; solution that will do everything.
As the Database Administrator, you have to know and understand each one of them.

I want to schedule backups with the app.

Scheduling needs a scheduling program.  Almost any one will do.
Backing up each DBMS needs the utilities for that DBMS.
Restoring any database needs the utilities for that DBMS and is, usually, a manual activity.