morti

Morti is strictly intransitive: you die, you cannot die something, and it never takes an accusative object. To kill is mortigi, with the -ig- suffix that adds a doer, and mortiĝi does not exist because the verb is already the change of state. Antaŭ tri jaroj is the standard way to say three years ago. The participle mortinta means dead in the sense of having died, against senviva for lifeless.