Well the first sentence in both of OPs links clearly says this is only two violinists, not three.

Snark aside, could the supposed "blue slip" be the blue valet tag? Usually used for F carry-on bags to valet, one of the other bullets on the back is musical instruments...I've gotten a blue tag every time I've traveled with my bass guitar on RJs.

If this turns out to be a overhead bins full/no more room, then a strong case for PreferredAccess for non-elite musicians. If you have something that needs to be in the cabin instead of the hold, pay for early boarding.