Will running 3 computers on the same internet connection slow the computers down?

Currently we are running two computers through comcast, the computer downstairs is hooked up to the modem, and my computer up here is hooked up wirelessly.. so far i've had no problems (unless my sister downloads music, then I really have a slow connection) but now my other sister is hooking up her laptop wirelessly to the connection..

plus I am furthest away from the modem.. will I have a slow connection now?

Will running 3 computers on the same internet connection slow the computers down?antivirus downloads

First, I'm going to assume your question is pertaining to the speed of your internet connection, not your computer. There is no way that the number of computers on an internet connection will slow your computer down.

Three computers in theory should not be any problem at all for a cable internet connection. More importantly than how many computers are on your network that feeds in to your internet connection via your modem, is WHAT you are you doing on those computers.

Your connection is alotted a certain amount of traffic; if you exceed or start pushing that traffic, your connection will drastically slow down. While surfing the internet or chatting will not do much to your traffic, since they are sporadic uses of your connection, stuff such as your sister downloading music can use up much more of your traffic.

Another thing to keep in mind is that the amount of traffic alloted for downloading is much more than that for uploading. Uploading files can drastically slow down your internet connection. So if your sister is downloading music off of a torrent program, or a program like limewire where she lets other users download from her once she has downloaded the files, that can also drastically slow down your computer.