I can tell you where you can find dozens of 3 legged red top ladders.
In our dumpster.
I agree with you that 3 legged ladders are much more stable than a 4 legged ladder. That much is true.
However, we had used red top almost exlusively for our step ladders from about 99'-03'.
Then from in 03'-04' we had 4 very serious injuries, all on three legged red top ladders. They have two serious design flaws in their step ladders.
1st, the spreader lock out is simply the flat bar bent over at the joint. The "ears" being bent over the other bar at the joint stop the spreader from pushing through. There is no other positive stop on the spreader. Once the ladder gets used for any amount of time the spreaders get loosened up and them employee will lock the ladder out, and the lock will let the ears slide past its locking point, making the ladder fold up on itself.
2nd, the third leg is held on to a piece of flat stock by a "U" bolt under the literal red top of the ladder. It is in a position that an employee cannot easily inspect it. You need to use an extension on a 1/4" ratchet with a wobbley to get access to the nuts to tighten them down. Without inspecting or tightening the nuts, the third leg loosens at the top, and it lets the leg slip up an out of the top. Again, down goes the worker.
We've paid now over $890,000 in workmans comp, and lost time wages strictly due to red top ladders. In 05' we scrapped every red top ladder we owned (approximately 70-80). We replaced them with both Red Bull fiberglass 3 legged ladders, and some Werner fiberglass ladders. We spent another $30,000 on the new ladders.
One of our injured employees is going to file suit against red top.
The moral of the story, I agree with you 3 legged ladders are the way to go, but don't go anywhere near a red top ladder. They alone should be outlawed, and I have no doubt they'll be bankrupt sometime in the next 10 years, as someone is going to sue them for everything.