Most don't - most have twin countershafts. The only transmissions I can think of offhand with triple countershafts are the proprietary Mack transmissions.
For brand new heavy duty trucks, try Heavy Duty Trucks USA. Trucks Junction and 1 Used Trucks both sell used heavy duty trucks online for a cheaper alternative.
trevass
The services offered by Freightliner trucks are the sale of heavy duty trucks, parts and service. They manufacture the chassis, heavy duty trucks and semi-trailers.
You can find heavy trucks anywhere from searching online to going into a car dealership/shop itself and looking around at different heavy trucks that they have available.
Thunders are good Independent trucks are good but are very heavy i don't think thunders have 20 dollar trucks and i wouldn't want heavy trucks
Here is a site with info on used heavy duty trucks and even trucks for sale: http://www.1usedtrucks.com/heavy_truck_subcategory.php. You can also check Craigslist for trucks in your area.
Heavy box trucks can be found in used car lots in rural areas. Also many manufactures like Ford carry a line of heavy box trucks for use on farms. Checking you local truck dealers will be the best way to find these trucks.
Depending on the types of trucks you are looking for, you can find trucks at http://www.penskeusedtrucks.com/. You can also find trucks at http://www.buyatruck.net/
The 4500/5500 trucks are assembled in Satillo Mexico.
Heavy trucks, trailers, industrial equipment, and heavy machinery.
low gear ratios
Means heavy vehicle