I have a Windows Desktop, a IBM Thinkpad Windows Laptop and a Macbook pro.
I want to share the keyboard and mouse between all three. Is this possible using a KVM Switch?
Yes it's possible. As long as the keyboard and mouse work correctly on all three computers when connected directly they should via a KVM.
I've used a two port IOGear KVM to switch between an Mac and PC.
You might also want to look into using a software KVM.
For example, I used Synergy for a while. It was really hard to configure, particularly on the Mac, but when it worked it was awesome. Basically you can move the mouse seemlessly between the different monitors to control each machine.