Bear with me. My heart is in the coffin there with Caesar, and I must pause till it come back to me.
I thrice presented him a kingly crown, which he did thrice refuse. Was this ambition? Yet Brutus says he was ambitious, and sure he is an honorable man.
Mark Antony is boosting up Caesar's credibility showing he was a good friend to him. He shows Caesars' loyalty, and by saying this he is showing admiration for his friend, in attempt to make the citizens feel the same way (3.2.48).
Mark Antony is showing his love for his dear friend Caesar by saying that because of his friend's death, he's in deep sorrow and he misses his friend. He does this to strike compassion in the hearts of the Roman citizens. He wants them to feel saddened by Caesar's death in order to get them against the conspirators and on his side (3.2.48).
Mark Anthony is contradicting what Brutus had previously said about Caesar being ambitious. It is a fact that Caesar refused the crown thrice. He does this innocently, but still in a way that attacks Brutus' credibility (3.2.48).