I am planning to collect data from 3 different plant units which need 3 AF servers. What is the scenario if I install it on a single server?

If you mean installing AF on one server and accessing the DA from 3 plants, there are no requirements, other than your AF server being able to connect to the plant's DA servers.

If you mean installing AF on one server and accessing the DA from 3 plants, there are no requirements, other than your AF server being able to connect to the plant's DA servers.