By dictionary meaning there isn't one. There is one word that is close which is 'Credo'
Meaning (from dictionary) 'A statement of the beliefs or aims that guide someone's actions.'
One three letter word relating to anger and ending with O is ego.
One word is EGO as in "He has a large Ego" or "Her ego has flourished".
ego
ego
Ego
ego
Ego egg Age, ago
gas, ego, big
Some nouns that end with O are:avocadobanjobarriobongobuffalocargoDumboechoEggoegoflamingoGeoGeronimoghettogigolohaloherohoboJellokangaroolassoLOGOmangomojitomottonachoOhioOreopatiophotopianopiccolopotatoradiorhinosolostudiotacotangotattootomatotorpedotriovetovideovolcanoweirdoyo-yozerozoo
Ego
I believe you are referring to ego.
ado, ego, ago, two, woo, zoo, two, pro, goo