TBB,
You are looking at 3 very different companies. All of them with a certain market speciality. All of them regarded very highly by most people.
Cole's is unique. Rich Cole has a very special relationship with Beretta. He travels to the Beretta factory several times a year. A lot of times the Beretta Gallery gunsmiths will refer people to Cole's for certain work. Of the 3 shops you mentioned, Cole's is the most focused. You will become very familiar with certain Beretta products. They do work on the Beretta semi-autos as well as their o/u guns. All of this is mostly repair, enhancement work.
Turnbull's speciality is restoration. Some very expensive, in some cases one of a kind, firearms have been entrusted to Turnbull's. Their reputation is impeccable. You will be dealing with much more than shotguns. You will not be building guns, but may in some cases be making the occasional part. Hopefully, you will learn the art of case color hardening. With that skill alone, you could build a career.
The biggest part of the interview process is knowing the company you are talking to. You will not be coming to them with experience, but rather raw talent, a willingness to learn and a great work ethic. This means at first, don't be concerned if you are working long hours. Those long hours will be paid for years later with the talent and skills you have refined. This applies not only to your current career choice, but just about any career path you intend to follow.
Pete
