They began the day by adding 54 runs for two wickets to a first-innings total of 347 all out, Jamie Overton claiming career-best figures of six for 95.
After that it was all down to the bowlers, led by Tim Murtagh. He took four for 28 as Somerset were dismissed for 152 and then added four more as the Cidermen scrapped to 112 for five - still 83 behind. Peter Trego, on 58no, and Jos Buttler, with 20no, hold Somerset's slim hopes.
Nottinghamshire had the chance to build a big lead in their clash with Surrey at Trent Bridge.
They added just one run to their overnight score of 273 for nine, but rolled the Brown Caps for 207 in 76.2 overs. Five of Surrey's top six fell in single figures, but their innings was rescued by Gary Wilson (49) and Gareth Batty (41).
Nottinghamshire were 24 for one at stumps - 91 ahead - after Stuart Meaker bowled Alex Hales.
Adil Rashid continued his good form with the bat as he struck 68 not out to leave Yorkshire in a strong position against Warwickshire at Edgbaston.
Rashid followed last week's century versus Somerset with the White Rose's top score in an innings that reached 318 for seven by stumps - a lead of 190. Adam Lyth's 43 was the next best effort, while Tim Bresnan - drafted in for Richard Pyrah after missing the cut for England - hit 38. Boyd Rankin and Jeetan Patel both took three wickets.
At Derbyshire's County Ground a stand of 179 between Michael Yardy and Chris Jordan put Sussex in the driving seat.
The away team were 109 for five when the pair came together, with Yardy making a defiant 121no from 169 balls and Jordan falling eight runs short of a maiden first-class century. At close of play, Sussex were 328 for eight, 105 in front.