Lee
asp
dew
niv
emu eau ecu
One example of a three-letter word that starts with "a" and ends with "o" is "ago."
Dud
Evita
To my mind right now, there is no word of your request.
Erica
Dun. Something of an archaic term, thesedays.
The 3-letter poisonous snake is almost always "asp."
The only English words that fit those requirements are tee, the, tie and toe.