Northamptonshire dominate in Cardiff
James Middlebrook top-scored with Northamptonshire making 292 all out in their first innings - opening up an overall lead of 158.
And in their second innings sorry Glamorgan were reduced to 71-4 before finally ending the day on 96-4, still 62 behind.
Northamptonshire had resumed their first innings on 145-5, a lead of 11 after only 17.1 overs were possible on the second day.
Andrew Hall and Middlebrook batted for the first 90 minutes of the third morning taking the visitors past 200 to register a batting point.
But the sixth wicket pair, who added 60, were separated when Hall drove Mike Reed straight to Ben Wright at point. Hall went for 32 with Northamptonshire at 202-6.
Northamptonshire had reached 218-6 by lunch for a lead of 84 .
Glamorgan took the second new ball and Michael Hogan broke through almost immediately to have David Murphy caught at slip.
After a rain break Wagg claimed the last three wickets in the space of 13 balls as Northamptonshire were dismissed for 292.
First he bowled David Willey before trapping Stephen Crook lbw for eight and then having Middlebrook at slip by Jim Allenby.
Glamorgan openers Ben Wright and Will Bragg survived until tea with Bragg batting with Dean Cosker as a runner after not fielding for most of the day.
The openers batted well until both perished in the space of only five balls.
Bragg was trapped leg before to Willey before Wright edged a rising Hall delivery to Trent Copeland at first slip.
And Australian overseas Marcus North failed for a second time edging a good Crook delivery behind as Glamorgan slumped to 69-3.
That became 73-4 as Stewart Walters fell to a sharp catch by wicket-keeper Murphy standing up to Hall. Four wickets fell for the addition of only nine runs.
Allenby and Murray Goodwin, who scored three from 41 balls, took Glamorgan to the close without any further scares.