Bolens literature treats them as 3 different series.
A few things do cross over somewhat, mainly the snowblowers and front blades, but you still have to have the correct sub frame to mount them to that is particular to each series of tractor.
The medium frames require that you take off the PTO and replace it with a "mule drive" to handle the belt to the mower deck. I believe the other two are both shaft drive to the mower deck.
I suspect you could adapt a tiller from any one to another, if you were unable to find the correct tiller. The Q series attaches the tiller to the axle, the medium frame and the 5000 series have adaptor plates bolted to the frame to attach the tiller. I have had a difficult time finding a tiller for the medium frame or 5000 series that included the driveshaft and the adaptor plates.
In the last several years I have seen only a couple of each series for sale in my area. We had an active Bolens dealer here, but none of these show up often. I have had better success buying a package than buying a tractor and trying to fill in what I need afterwards, but you take what you can get.
The list of attachments for the medium frame tractors is much larger then for the other two series, but I sure don't see much for sale around here! I suspect that when a tractor gets sold, especially in an estate sale, a minimal amount of the parts and pieces get sold with the tractors, and the rest gets thrown away.