Lee
dew
niv
asp
emu eau ecu
One example of a three-letter word that starts with "a" and ends with "o" is "ago."
Dud
To my mind right now, there is no word of your request.
Evita
Erica
ern, also known as a sea eagle
Dun. Something of an archaic term, thesedays.
The 3-letter poisonous snake is almost always "asp."