chief = prince. Not by birth, but by merit. Probably "the chief musician".
was for song, or, was for service: especially the service of carrying up the Ark.
about the song. Three Psalms appointed: 1Ch_16:7-36. Psalms 5 and Psalm 11; which, being suitable for general use, were afterwards handed over to "the chief musician". See note on the subscript at end of Psa 3.