I think if you matched the number or used 3 wheelers and atv's being used, i think the numbers would fall in favor of injury on trikes. The number of deaths/injuries on these machines shouldn't be classified towards just atv/atc, but rather the ages of those injured. Most injured are inexperienced, younger riders who haven't fully gained the aspect of safe riding. Accidents do happen, but serious accidents fall towards those who ride inappropriately or careless, and with not much experience. (These are my opinions only, accuracy may be limited, or may be very accurate)
WARNING: I'm About To Do Something REALLLYYYY Stupid!!!
Past:
'82 Honda 110
'82 Honda 110
'83 Honda 185s
'85 Honda 200m
'85 Honda 250sx
'85 Honda Auto-X
Present:
'82 Honda 250r
'82 Honda 250r (Basket Case)
'83 Honda 200s (Parting out)
'84 Honda 200x
'84 Honda 250r (Basket Case)
'85 Honda 350x (currently being built)
'85 Honda 350x (waiting to be built)
Future:
'83-'87 200x's (wish list)
'86 Honda 350x (eventually)