This is why I think the Sinister Six won't be in TASM3. The plan from the beginning was to make a story arc that lasts three films, and I think everything is going to wrap up by the end of the third installment. However, with the announced S6 movie, there will most likely be some hints or "seeds" planted here and there to foreshadow "the future," such as the room with the six chambers or the Man in the Shadows being revealed as "the gentleman."
By the end of TASM3, we should have at least half of the villains needed for the Sinister Six. Also, for all we know, they are actually going with the Ultimate Six storyline where Peter is forced to be the 6th member.