I think it is called a Rocket 3 for the same reason a Harley Davidson is NOT called a R2.
I, for one and I'm sure most here will agree, think "rocket" is a pretty good description.
Truth be known the Rocket I aka the Titan I prototype never made it off the ground. The Rocket II lost too many passengers and the parachute wasn't DOT approved. Now you know the rest of the story...
How I remember it, BSA had been using the Rocket name for over 10 years on the 'high performance' A10 range - Road Rocket then Super Rocket. They just used the old Rocket name adding the '3' to signify 3 cylinders - as you say.
Not as 'pretty' as the Hurricanes though... I wouldn't say no to one of the older Rockets. Baxters in Maine had a couple for sale a while ago .. build one out of the two type deal , priced well I thought.