John Gill Commentary 2 Chronicles 35:15

"And the singers the sons of Asaph were in their place, according to the commandment of David, and Asaph, and Heman, and Jeduthun the king`s seer; and the porters were at every gate: they needed not to depart from their service; for their brethren the Levites prepared for them." — 2 Chronicles 35:15 (ASV)

And the singers the sons of Asaph were in their place
In the court of the priests, singing and playing on their instruments while the sacrifices were offering:

according to the commandment of David, and Asaph, and Heman, and
Jeduthun the king's seer ;
the same with Ethan; and everyone were seers, as Jarchi interprets it, that is, the three last:

and the porters waited at every gate ;
such of the Levites as were in, that post and office:

they might not depart from their service ;
to let people in and out, that came for their passover lamb, and share in their other offerings:

for their brethren the Levites prepared for them ;
because they were not at leisure to prepare for themselves.