The only issues I heard about registration on a M3W or Polaris Slingshot is from Texas, Connecticut and a couple other east coast states that do not know how to classify them.
I know in Texas you have to have a saddle that you straddle and handlebars. So even though it has three wheels, it has seats you sit in and steering wheel. I believe that a "few" M3W's got through in Texas, but when the Slingshot hit big last summer, they clamped down and refused to plate them.
I'm sure that Polaris has or is working on getting them approved.
If they are motorcycles, then all the "car" requirements go out the door and they may not be so picky on the brakes, tires etc.
Good luck with your purchase and now is the time to be getting it sorted out. I'm sure the dealer you purchased from knew you were a resident of Maryland and that there may be issues. I would throw this back in his lap for some assistance.
