Take a closer look inside first. Last summer I bought a Beomaster 4400, which also had one non-working slider. In this case, the actual potentiometer had moved out of place (it's just clipped there), and the fork of the slider didn't reach it. If the 3000 is similar, you might have an easy fix
I'm sure spare parts aren't available new, so you'll be looking for some kind of a glue fix if the fork of the slider (or the pot) is actually broken, or maybe someone on the forum has a parts unit.