In Act 3, after the assassination, the protagonist appears who drives the rest of the action through the rest of the play. Who is this person?What...

The character who comes to the forefront in Act Three is
Marc Antony.  After the conspirators leave the scene of the crime, Marc Antony plots his
revenge and then carries it out through the rest of Act Three.  He uses his ability to
command the language and his ability to persuade the commoners to get revenge on the
conspirators, specifically Brutus and Cassius.  Marc Antony is able to induce the
conspirators to action so he doesn't have to be guilty of an unfavorable
action.

He are prepared for Marc Antony and his tactics by
his brief appearance in Act Once, scene two.  He has few lines, but his loyalty to
Caesar is evident:  "When Caesar says do this, it is
performed."

However, I would argue that Marc Antony is not
the protagonist of this play.  The play centers around Brutus and his decisions and
consequences thereof.  Brutus desires are better Rome, and figures that Rome will be
better under the rule of himself or others, not Julius Caesar.  Marc Antony is what
stands in Brutus' way of establishing a new Roman republic.  However, an argument can be
made either way.