Glamorgan earned a first-innings lead of 19 runs, helped by a magnificent 130 by Marnus Labuschagne. At the close, Derbyshire were 170/2 in their second innings.
Labuschagne began day three looking to build on his overnight score of 53 along with nightwatchman Timm van der Gugten. Looking in fine touch, the World's number one ranked men's test batter went on to score 130 to help Glamorgan to a total of 387 and a first-innings lead of 19 runs.
Glamorgan took two wickets before the close, a wicket apiece for van der Gugten and James Harris as Derbyshire ended day three on 170/2.
"I'm pretty happy with how I batted," said Labuschagne. "Obviously as a batter you're never satisfied, it was frustrating to get out there and not be able to add to that lead which would have been nice. All in all, it was nice to get a big score and get us in a position where the game's on for the last day."
"As we know, we're in the UK so anything can happen, it can happen very quickly. The wicket, towards the back end of our innings, went quite low and the stumps came a lot more into play. Obviously, the heavy roller took a little bit of the sting out of the wicket with our new ball and they scored a little bit more freely."
"I think if we get a couple of wickets tomorrow and put them under, we could have a hopefully around 230-240 chase which would be a really good game. But it's hard to predict where the game's going to go and we're going have to just take it ball by ball."