Jamieson, Fausset and Brown on Isaiah 6:8

8. I ... us--The change of number indicates the Trinity (compare Ge 1:26; 11:7). Though not a sure argument for the doctrine, for the plural may indicate merely majesty, it accords with that truth proved elsewhere.

Whom ... who--implying that few would be willing to bear the self-denial which the delivering of such an unwelcome message to the Jews would require on the part of the messenger (compare 1Ch 29:5).

Here am I--prompt zeal, now that he has been specially qualified for it (Is 6:7; compare 1Sa 3:10, 11; Ac 9:6).