There are two ways I can view your question. One is going to be far more trouble than it's worth, the other, doable, but you probably won't be happy with the results.
The far more trouble than it's worth interpretation, is if you want to be able to watch "live" TV broadcasts on the other systems. The best way to do that would be to get TV tuner cards for each system, and run some cable to each. With digital cable, this might involve having to get some extra decoder boxes, unless you're fine with restrictions on how many different channels can be viewed at a time.
If you want to record a show using the media center system, and then be able to stream the recorded show, via a LAN, that's at least theoretically doable. However, it depends on how XP media center stores recorded files, and whether or not there's any easy way to make those files viewable on non-media center systems. If it's straight MPEG-1 or MPEG-2 encoding, you're good to go, but if it uses some proprietary format, you may be forced to use other software or buy one of those media streaming kits.
I suspect that this whole process is going to be well beyond your abilities though, even if it turns out to be possible. You might want to just investigate other options. A good one is getting a 4th system which will become a MythTV box. You can use it to record shows, in a widely supported format, and then easily stream those files from system to system. It could be an aging system from the Pentium 2 days, since all it really needs is a supported TV tuner card, and lots and lots of disk space. You don't need much in the way of CPU power or RAM if you never intend to watch the recorded shows on the MythTV box. It wouldn't even need a monitor once you get it all up and going. You should be able to control it remotely over a network with minimal hassle.
Currently have digital cable & internet, want to connect media center computer to 2 other computers in house to watch tv on them. Do I have to connect cable box to computer first then to other computers?