[BillyCoxDodgers3B]

They most certainly would authenticate it, but would make note of the clubhouse Murtaugh signatures (in addition to any other clubhouse signatures that may be on the balls). Of course, there's still good value there with all three Clementes being authentic, in addition to every other signature in the photos you shared. Someone else signing on Murtaugh's behalf was far more common than not during this era. While the two I can see in your photos were definitely written by the same person, there were at least a few different people signing baseballs on Danny's behalf back then! Actually, Pirates balls from the 1960's & 70's are notorious for clubhouse issues and are not limited to Murtaugh or the star players. Even the more obscure guys were targets. To find 3 with authentic Clementes is great!

The Pirates and Tigers were to clubhouse signed balls in the 1960's & 70's what the Dodgers were in the 1950's. To find a team ball completely devoid of any clubhouse autographs from these eras is quite rare indeed. The other teams all had some issues with this to a point, but it's been my experience that the teams mentioned were the most active at the time in this regard.

My advice would be to not pay extra for the grading (signature strength) if you're planning on having them authenticated. If you're selling them, just consign to an auction house like REA or Hunt and save yourself the time and money of having these authenticated. Auction houses Heritage and Goldin charge for the authentication, so I'd personally go with one of my two suggestions to avoid this.