Standard FFPC league are dual flex allowing only for RB, WR or TE as flex options. I don't play these leagues myself so someone correct me if I'm wrong but I believe FFPC SF leagues are dual flex with one flex allowed to be a QB. Triflex means 3 flex spots with one allowed to be used as the QB.
Typical FFPC dynasty leagues (both 1QB and superflex) require kicker and defense, and have a max of two flex spots. So an ideal superflex team would start a second QB in one flex, and the other flex would be for RB/WR/TE. Because these leagues require teams to start two RBs and two WRs, a big complaint is that RBs are too overvalued. (I have found this to be the case, especially in 1QB leagues.)
The Triflex format is superflex but eliminates the kicker and defense and replaces those positions with an additional starting WR and additional flex. So teams are required to start two RBs and three WRs, and an ideal team would start a second QB in the superflex spot but still have two more flex spots to fill with non-QBs. The format elevates the value of WRs (because teams could start two QBs and five WRs) and puts them on par with RBs. It's my favorite FFPC format because it opens up so many other trade scenarios.