Electric motors are very good for towing due to having tremendous torque. If there happens to be a Model ≡ 100, it will likely have tremendous towing capability.
You're missing the point. There is a LOT more to towing than torque, as I posted above. You need a match between motor torque (which I carefully left off, as that one's easy), suspension, frame ("carriage") strength and structure, and transmission.
My takeaway point was and is, however, that given the rather significant restrictions of a $35K sales point, there are concomitant restrictions to what Tesla Motors can provide. Give the Model 3 the characteristics needed to be even a modest tow-er, and other features in far greater demand will perforce be omitted.
Now, as to whether one then can provide such ability as an add-on? I don't believe so. In my experience, it is not possible appropriately to beef up a carriage to make what almost certainly is a lightweight vehicle truly able to tow within any reasonable price - you're talking about a completely different frame. Suspension? The multi-link rear suspensions that Tesla uses are an incredible engineering feat, which means the effort would need be duplicated - thus twice the upfront creation cost - to multi-purpose the vehicle. Transmission? Here the advantages of an EV mitigate - but do not absolve completely - the cost problem, as that more readily can be beefed up as necessary.
Are those costs the appropriate ones for The Company Maker - which is how I believe the Model 3 must be viewed? I absolutely, incontrovertibly, cannot see it.