What does Antony feel is Lepidus' role in the triumvirate?
Octavius, Antony, and Lepidus form a triumvirate after Antony's oration at Caesar's funeral has created rioting which forced Brutus and Cassius to flee from Rome. Lepidus' full name was Marcus Aemilius Lepidus. According to Wikipedia:
Though he was an able military commander and proved a useful partisan of Caesar, Lepidus has always been portrayed as the weakest member of the triumvirate. He typically appears as a marginalised figure in depictions of the events of the era, most notably in Shakespeare's plays.
It was because of Lepidus' strong relationship with Julius Caesar that he was brought into the triumvirate. Antony always regards Lepidus with contempt, both in Shakespeare's Julius Caesar and in his Antony and Cleopatra. In Act IV, Scene 1 of Julius Caesar, the three members of the triumvirate are shown deciding upon which conspirators and political enemies are to be killed. Then Antony demonstrates that he considers Lepidus an inferior member of this triumvirate by sending him on an errand. He tells Lepidus to go to Caesar's house to fetch the will so that they can all consider how to get out of paying some of Caesar's bequests to the Roman people. This shows a callous side of Antony after his tearful tribute to his fallen friend in his great funeral oration. After Lepidus leaves, Antony expresses his contempt to Octavius in full.
This is a slight, unmeritable man,
Meet to be sent on errands. Is it fit,
The threefold world divided, he should stand
One of the three to share it?
A barren-spirited fellow, one that feeds
On objects, arts, and imitations,
Which, out of use and staled by other men,
Begin his fashion. Do not talk of him
But as a property.
In Act II, Scene 2 of Antony and Cleopatra, Lepidus is made to appear a toady, trying to placate both Antony and Octavius and persuade these two powerful men to reconcile after their falling out over Antony's relationship with Cleopatra. And in Act II, Scene 7 of that play, Antony will treat Lepidus like a fool. For example, when Lepidus asks about the Egyptian crocodile, Antony tells him:
It is shaped, sir, like itself; and it is as broad as it hath
breadth: it is just so high as it is, and moves with it own
organs: it lives by that which nourisheth it, and the elements
once out of it, it transmigrates.
Eventually it will be Octavius who will strip Lepidus of all his offices except that of Pontifex Maximus and send him into exile in Circeii. Lepidus is a good general but a weak politician.