Know Your Rights and Limitations When You Fly
So it's great that the TSA allows photographers an extra bag of "photographic equipment in addition to one (1) carry-on and one (1) personal item through the screening checkpoint. The additional bag must conform to your air carrier's carry-on restrictions for size and weight."
The catch is that your airline, not the TSA, has the right to disallow the third bag. While you can get it through security, you may be forced to check it at the gate. Since many photographers need two bags for photography equipment plus a briefcase for their laptops, you may not want to have to choose which bag is thrown below when the third bag is disallowed.
E-mail messages to Delta, Northwest, Alaska, United and American Airlines asking whether they allow the extra photography bag either were not returned or the responses quoted/referred to baggage guidelines from their websites that allow only two pieces of carry-on luggage. Even if any of the replies had been positive, the rules may be applied differently at the gate.
The safest plan is to go with two carry-on bags, at least for now.
Take my advice; get professional help.