On day 3, you surely would not hang around Thornbury Castle Hotel for a whole day? My advice would be to drive on back roads to the M48 Severn Bridge & leave M48 for A466 at first junction on Welsh side of estuary - then drive into Chepstow to see the medieval castle. (Look closely at the road pattern - which is complicated in Chepstow). Then leave Chepstow & take A466 going N. Just after St. Arvans - a single lane road comes off the AA66 and about 3 minutes drive it has a sharp corner with a rough parking area on R. Park up & follow trail up through the woods for 17 minutes & turn R where it levels off to see Eagle’s Nest Viewpoint. Then back to car & continue up A466 to see ruins of TINTERN ABBEY.
The A466 continues to be scenic going N up the Wye Valley. Yet Rock viewpoint is also well worth a visit or alternatively head west at Monmouth on A40 to Raglan and then west towards Abergavenny. Go up steep hill to Blaeavon and take kids town the Coal Mine at BIG PIT. (Free). On returning to Severn Bridge - go to Raglan - see Castle and then down A449 expressway towards Newport and then east on M4 & M48 to cross the estuary back to England.
Map of bridges:>https://firstname.lastname@example.org,-2.6865857,12z/data=!5m1!1e1
Map showing Monmouth to Raglan to Blaenavon for Big Pit:>https://email@example.com,-2.9681038,13254m/data=!3m1!1e3!5m1!1e1
PS. If you wanted to take the train to/from London(Paddington) to Thornbury - you could do it in around 1 hour as most of the line is fast = 125 mph to BRISTOL PARKWAY station. Then hire/ditch car if possible. Advance book specific train for cheapest fares around 11 weeks ahead. www.nationalrail.co.uk
These are the trains (green) :> https://www.youtube.com/watch?v=CGXboGzZiXI
If driving from Thornbury to Bath - best to use Lansdowne Park & Ride. (A P&R is where you park the car on the edge of a city and shuttle buses take you downtown. They have a similar thing at Oxford. Remember, these are old places built long before we had cars.