How to realize multi-devices restaraunt manager

I’m working on application for managing restaraunt’s orders.

Actually I made a simple version that works independently (written in C#). It is able to take orders (take-away, home delivery or table-order) store them, printing tickets for the kitchen, make statistics and so on. I would like to realize the possibility to manage the database from several terminals (actually the device is a tablet with windows 10) so from another tablet or smartphone so the waiter can take the order directly from the table.

I haven’t never develop anything similar, my idea was to create a server that has a database (what is the best for this application? MySQL?) so other devices has to interact with it with some protocol (SSH?). Is this a good solution? If I would to use 3 devices (one tablet near the cashier, two smartphones) I need to use a fourth device dedicated for the server? If so I thought about a Raspberry.

Thank you if someone can give me just an idea to start the develop.

Good day to all.