I would say that most documentary photographers focus on people as subjects, and are at least familiar with the work of those three, and many others. As far as being "influenced by", they certainly are three influential photographers, in general!
I personally lean toward documentary and journalism when left to my own devices. In the projects so far, probably about About 75% have been about people, but more based on artifacts than on their actual appearances or activities. The other 25% have involved shooting actual pix of people, alive and moving. I love to examine what is left behind, either via abandonment or death.
Personally, Gene Smith is probably the most influential of those three on me. Although I would not necessarily list him as a big influence, I do enjoy much of what he did stylistically and aesthetically. Mary Ellen Mark I could take or leave (interesting subject matter, but not extremely photographically/visually interesting to me). Sebastio Salado I just do not care for at all, aesthetically speaking.
When it comes to influences, I don't have many with documentary photography. My influences are more journalistic and "street" oriented. I am actually quite fond of Ansel Adams' small body of documentary work. Lee Friedlander and August Sander are two big influences, although I wouldn't say I take pix anything like Sander. I just appreciate his ability to perform such vast undertakings, while mixing the subjective and the objective quite seamlessly, and to do it all beautifully in a visual sense. If I am lucky, I can take pix 1% as good as Friedlander's.
Why do you want to know?