A1. Steps, Mister? Outside? Thank you (5)
A2. Aerial exercise outside, computer network inside (5)
A3. Canonized hollow fruit recurring (6)B1. Light-hearted student at first sent back in the Middle East (5)
B2. Tom and his family give captain's head injuries, topless (7)
B3. In tax office I blossom (4)C1. Stunning suit (4)
C2. Explorer follows gold pens (6)
C3. Reformer reportedly menaces Metropolis (6)D1. Faux marshes contain radioactive material (5)
D2. Not objecting shortly returning award (4)
D3. Man removes butt from rifle, briefly asks for reason (3)
Looking for four place names, one for each triplet. Each answer is clued with the same (mysterious) method.
Note: The part of speech required for each answer is listed below. As pointed out in the comments, these don't agree with the definitions in many cases... My bad!
A1. singular noun
A2. singular noun
A3. singular noun
B1. singular proper noun
B2. plural noun
B3. singular noun
C1.singular noun
C2. singular noun
C3. singular proper noun
D1. adjective
D2. singular proper noun
D3. singular noun