You don't need origin stories for villains like Shocker and Rhino. We are already being introduced to Rhino, and does he really require much of a backstory? No. In the MCU, we had solo movies for Iron Man, Hulk, Thor, and Captain America. Then Hawkeye & Black Widow were added to the Avengers without much backstory.
We have Electro and Rhino in TASM2. I'm not counting Lizard because, well, he wouldn't join the Sinister Six. So that is two down, four to go. They could add Shocker and Rhino to a S6 film while using other villains introduced in previous films who have already been developed.
TASM3 will most likely have Green Goblin, since that is what Marc Webb is leading up to. But GG is not a S6 villain, so lets not count him either.
There is a recent report (was this from comic con?) that talks about there being a "mini movie" after TASM2. I'm assuming they're talking about a post credits scene, similar to all of the Avengers movies.
If so, I could see this being where the Six are formed, or at least started to be formed. Could easily see something where Gobbie talks with Toomes, or breaks Rhino and Lizard out of prison, or contacts Kraven or Black Cat, something along those lines...