How do I fetch data from my multiple websites databases and display it in main website?

Let's consider I have three separate websites for 3 different departments with different database, servers etc.

And also I have a main website where I want to get some data from all three websites and show it in main website.

E.g. In my main database if I want to get all users then it should query each of the 3 different websites, get the content and display it on my main website.

What approach should I use to achieve this ?

I am not asking for the code and queries. I only want to know the approach.

via Chebli Mohamed