Living in a joint family we have 3 internet connections, each floor has one internet connection which is split per floor. Each internet connection is shared between 4-5 computers using a switch per floor. Each of these internet sharing networks are independent of each other.

What I want to achieve here is a local network (for local messenger and file sharing) that can combine all the 3 independent networks, problem is that whenever I try to do that the whole network tends to use just one internet connection. I have all the necessary hardware.

How do I solve this problem, one approach could be that I could get one PC to act as a server and bridge the internet connections and then the whole network would have to access the internet through this server. Theoretically this could be possible but I have never tried this approach in real life.

Also if certain computers need be restricted from internet access how would this be possible on the same network?