I came across the E-3 AWACS aircraft with the designation E-3, I could not find what the letter E means in E-3.
The E isn't exactly the short form of a word as it is a mission designation. In the US military's Tri-Service aircraft designation system, E indicates that the aircraft performs special electronic missions. Similarly, F implies the aircraft is a fighter aircraft and B indicates the aircraft performs bombing missions.
edit: here is the US military aircraft designation system