No real need for a server and if you did need one then you would know you needed one.
To use a server you could use Active Diectory to control the PC's but I wouldn't even bother.
You could setup a file server at least to store data on and any of the 3 PC's could access the data. But there are are more easier ways to this with NAS plus it's smaller and cheaper.
For your websites you would need a static IP address and one that your ISP would have to issue you [if they issuse them]. You might have to swap ISP to do this. Hosing these days is cheap [depending upon your requirements].