Adam Clarke on 2 Samuel 20:4

Then said the king to Amasa - Thus he invests him with the command of the army, and sends him to collect the men of Judah, and to come back to receive his orders in relation to Sheba, in three days. It appears that Amasa found more difficulty in collecting his country-men than was at first supposed; and this detaining him beyond the three days, David, fearing that Sheba's rebellion would get head, sent Abishai, who it appears was accompanied by Joab, to pursue after Sheba.

Amasa, it seems, got up with them at Gibeon, 2Sam 20:8, where he was treacherously murdered by the execrable Joab.