I do think it is pretty lame needing a 3 button mouse for fluxbox. I though Linux was supposed to be good for old harware too.
Even more annoying when you are using a laptop, as they tend to not have a middle mouse button and just have a scroller button. I think you could change them in Winblowz but not helping here.
You have got a valid point though, as none of my PS2 mice have a third button, the only mice I have that have a middle mouse button are the ones that are USB and of course, they are all fairly new.