There is no such thing as "conduct unbecoming a Marine" under the Uniform Code of Military Justice. The closest thing to it is what is known as "Article 133" (Conduct Unbecoming) which could carry 'dismissal' as a possible punishment, since it's only applicable to Officers (Bad Conduct and Dishonorable discharges only apply to enlisted personnel).
As ENLISTED personnel what the third charge SHOULD have been was Article 128 - Assault Consummated by Battery, which carries a custodial sentence usually followed by a Bad Conduct Discharge.
Most cases of "Dishonorable Discharge" heard about in media (papers, TV, Movies) are actually Bad Conduct discharges, since the only charges that carry a dishonorable discharge are the most serious ones (Attempted Murder, Murder, Sexual Assault, Desertion, and Treason)