What is the temperature and pressure of the exhaust that comes out of the rocket during the last three seconds of countdown (smoke that stays near ground)?
How much carbon dioxide is there in that smoke (approximately)?
The temperature and pressure inside the engine's combustion chamber is very high -- in the ballpark of 3400º C and 100 atmospheres for the Falcon Heavy's Merlin engines.
However, the bell-shaped nozzle of a rocket engine expands the exhaust stream, which both cools it and reduces its pressure. Ideally, for best performance, you want the exit pressure to match the ambient air pressure you're exhausting into; at sea level that usually means the exhaust is actually below ambient pressure because it's optimized for higher altitude flight.
The exit pressure for Merlin is about 0.7 atmosphere; I'm not sure about the temperature but it's probably around 1500ºC.