McFadden, 29, accepted the offence and an initial five-week ban was reduced to three after his past disciplinary record was taken into account.
The suspension rules McFadden out of this Friday's Pro12 game at Ospreys.
The Ireland player will also miss the Champions Cup game with Bath at the RDS and the following week's trip to Wasps.
McFadden appeared before a disciplinary committee in Edinburgh on Wednesday after being cited for stamping or trampling on Connacht's Nathan White during a ruck.
A Pro12 statement said: "Mr McFadden accepted that he had committed an act of foul play and that it would have warranted a red card."
Leinster back Fergus McFadden has been given a three-week ban following a stamping incident in his side's Pro12 win over Connacht on New Year's Day.