I've looked around for hours, but I cannot seem to find what I am looking for.

I have a D-Link DIR-655 Router connected to a cable modem.
I have 3 Linux boxes (2 Gentoo, 1 Fedora Planet CCRMA (soon switching to Gentoo w/Pro-Audio Overlay)) hard wired into the router.
My wife has a Windows laptop with a wireless link to the Router.

All have Internet access.

I also have a Brother HL-5250DN Network capable printer.
It has an Ethernet port, not currently used.
The Printer is about 5 feet from my router, so adding an Ethernet cable would be no problem.

The Brother Printer is connected to my "Main" Gentoo computer via USB cable. I'm using cups. It works good.

What is the most simple (or, if not simple, the best) way to:
1) Share files between computers (most desired function)
2) Have all computers share the Brother HL-5250DN Printer
3) Be notified when the Windows machine transfers a file to my "Main" computer

The most important is to be able to share files between the Linux Boxes, but I still need to be able to share files with the Windows laptop as well.

And if the instructions for the above scenario can be clearly laid out, that would be great!

Thanks in advance.

Use SAMBA on one of the linux boxes. There's TOO MUCH information on the web about setting up SAMBA shares to recommend anything here. I've not checked, but I bet there's loads on this forum.
It does all of what you want (with the possible exception of the notification. I suppose you could increase the debug level for the log, and then have cron, or some other daemon check it and report).