In the language of the armed forces, 'coming in hot' means that their weapons are set to fire. In many weapons - missiles, cannons, etc. - there is a switch or mechanism that arms the device and makes it ready to fire. In any sort of a normal (non-combat) situation, these devices are turned off for safety - they are 'cold'. In a combat, or suspected combat situation, they would be set active, ready to fire - they are 'hot'. In the above sentence, they are coming in with all weapons 'hot' and set to fire.

It means the vehicles are approaching at very fast speeds, or in a reckless manner.