Need advice, which is the best option for a “service”

We have 3 locations in different cities. Each location has their own client list.

The client balance calculation rules sometimes changes. And the calculation takes some time.

So my task is to create a single solution for all 3 locations for the client balance calculations. Basically, the business logic should be one code shared by all the locations.

Which is the best option for performance and sharing ? a combo of web service and a internal library for calculations ?

Lets say, c# code for the calculations (gets and returns a json, makes calculations+database) and maybe php for the webservices/interface with the locations?

or maybe everything on php ? or it's faster c# + sockets ?

or a micro service on AWS (again, all in php? or c# mix) ?

help!