As far as I know, I have only 3 devices connected to my iTunes match, and my iTunes on my work macbook tells me that I've used up all 5 of them! Is there any way I can look at what those 5 devices are, so that if there really are TWO extra devices that I don't know about, I can at least take them off?
The limit for iTunes match is actually 10 devices, but the limit of 5 computers that can be authorised against an iTunes account also applies here, and it sounds like that is what you have come up against here.
In iTunes, go to the store and then click on your iTunes account name towards the top right. When you enter your password you will get your account information. You can then see what computers are authorised on that account.