Final answer:
The three hazard classes not to be loaded into a heated trailer are explosives (Class 1), flammable gases (Class 2), and flammable liquids (Class 3) due to the increased risk of fire or explosion.
Step-by-step explanation:
The three hazard classes that should not be loaded into a trailer with a heated air conditioner unit are the following:
- Explosives (Class 1) - due to their potential to detonate, especially when exposed to heat.
- Flammable gases (Class 2) - as they can ignite or explode when exposed to an ignition source like a heated air conditioner unit.
- Flammable liquids (Class 3) - these can vaporize and form combustible mixtures with air when heated.
Loading these materials near a heat source increases the risk of fire or explosion, jeopardizing the safety of the transport and surrounding area.