A1A-3
"A1A-3" means there are two trucks. The "A1A" truck is under the front of the unit, and has one powered axle, one idler axle, and one more powered axle. The "3" truck is under the back of the unit, and has three idler axles in a row. An example is the Baldwin DR-6-2-10 1,000 hp (750 kW) cab unit, only one of which was built for the Chicago and North Western Railway in 1948.
Read more about this topic: AAR Wheel Arrangement