Sforno on Numbers 3:8/ספורנו על במדבר ג׳:ח׳

1משמרת בני ישראל לעבוד את עבודת המשכן, the auxiliary aspects of the service in the Tabernacle, to carry, to sing, etc; these tasks, if not for the sin of the golden calf, would have been assigned to ordinary members of the people at large instead of being reserved exclusively for the Levites.