It's kinda impossible to tell cause second season has been skipping a lot of chapters. S1 covered ~50 episodes in 20 episodes, S2 ended on chapter 106. IIRC things started moving at chapter ~160 or so so one would need at least another season to get to that material.
I don't think I could stand a third season of absolutely nothing happening... that's even less plot-to-filler ratio than Detective Conan! Plus, having to release another season of mostly filler episodes would probably lower the chances of S4 ever happening.
...Then again, I'd rather have studios work on other IPs rather than spend their time on churning out more Nisekoi.