However, the ban could be reduced to a fortnight if the player completes a coaching course.
Nel will definitely miss the Challenge Cup last-16 tie against Bath this Saturday and the game against Zebre in the United Rugby Championship on April 22.
But if he applies for and completes a World Rugby Coaching Intervention, he will be free to play from Monday, April 25. That would mean he would be available for the URC match against Ulster on April 30.
Nel, who came on as a first-half replacement in the 54-5 win against Pau, was cited by the match citing commissioner Dana Teagarden of Germany for striking the head of Bordenave, in the 42nd minute, in contravention of Law 9.12.
The law states a player must not punch or strike with hand, arm, elbow or shoulder. Under World Rugby’s Sanctions for Foul Play, Law 9.12 carries the following tariffs - Low End: two weeks; Mid-range: six weeks; Top end: 10 to 52 weeks.
An independent disciplinary committee chaired by Simon Thomas of Wales and also comprising Donal Courtney (Ireland) and Martyn Wood (England), heard submissions from Nel, who accepted the charge, from the player’s legal representative, Bruce Caldow, from the Edinburgh Rugby team manager, Matt Cornwell, and from the EPCR disciplinary officer, Liam McTiernan.
The committee upheld the citing complaint, finding that Nel had struck Bordenave’s head with his shoulder and determined that the offence was at the mid-range of World Rugby’s sanctions and six weeks was selected as the appropriate entry point.
But on the basis that the player accepted that he had committed an act of foul play and had a clear disciplinary record, it was decided to cut the ban in half and impose a three-week suspension.
Both the player and European Professional Club Rugby have the right to appeal the decision.