It would not be difficult (depending on your comfort level with connecting wires), but it may cost you a bit more money. Presumably it is not a USB model, which means it will need to be connected first to an optical interface, which can then be connected to your USB port.
You can make your own optical interface by disassembling a USB or PS2 mouse and connecting the trackball to the encoder board inside. Search the forums and wiki for more information and examples on this method (commonly called a mouse hack).
Or you can buy an optical encoder. Groovy Game Gear sells one called the Opti-Wiz, and Ultimarc sells one called the Opti-pac. Or if the I-Pac is not already purchased, you can buy the Mini-Pac instead (also from Ultimarc) which is a combined I-Pac and Opti-Pac all in one. Lots of "pacs", I know.
And 3" is very common in games. Centipede used the smaller 2.25" trackball, and Missile Command used the huge 4" trackball. But most people use the 3" if they have the space.