Not knowing whether you have a low-normal or high-normal derailleur, and without covering the whole derailleur adjustment process in detail, do this:
Unscrew (CCW) the barrel adjuster 1/2 turn. Shift through the range of gears, being sure to switch between the small and middle rings of your crankset.
Continue unscrewing in 1/2 turn increments until the jumping chain goes away. You shouldn't need more than a couple of turns.
By unscrewing the barrel adjuster in these 1/2 turn increments, you are effectively lengthening the housing, which effectively shortens the cable. This process addresses any cable stretch (or housing / ferrule compression) that may have taken place in the time since the shifting was last adjusted.
Screwing the barrel adjuster in (CW) would have the opposite effect, "lengthening" the cable.
If you're interested, check out SheldonBrown.com or ParkTool.com for detailed step-by-step instructions on derailleur adjustment techniques.