The industrial answer is to make the perimeter that encompasses the OD of each pulley and the tangents to those edges equal to the inner perimeter of the belt, so that it fits over all pulleys at the same time. Then, either cam one shaft into location (ie - slide) or use a cam-style idler roller for belt tensioning (cam followers work great for this).
Of course, if you don't want to have a separate tensioner and you want the shafts to be fixed in place, you can have one of the pulleys have either no flange or a removable flange. After all, you typically only need flanges on every other pulley.
A really good resource for timing belt information is
http://www.martinsprocket.com/2001/SecK.pdf#J2
It has center-to-center charts, rules of thumb, engineering calculations, and design criteria.