Ireland's Sexton free to play in Rugby World Cup despite ban
But the 38-year-old fly-half will miss next month's warm-up matches against Italy, England and Samoa.
Sexton faced a disciplinary hearing over "misconduct complaints" after exchanging words with referee Jaco Peyper following his club Leinster's loss to La Rochelle in the Champions Cup final in Dublin on May 20, a match the injured Sexton did not play in.
The charge can carry a long ban, but a statement issued by European Professional Club Rugby on behalf of an independent disciplinary committee said: "Having regard to the full circumstances of the misconduct and the mitigation which is set out in detail in the written reasons, the disciplinary committee concluded that the appropriate and proportionate sanction was an immediate suspension of three matches.
"He (Sexton) is suspended with immediate effect and cannot play in the following Ireland matches: 1. August 5, Italy; 2. August 19, England; 3. August 26, Samoa."
Leading contenders Ireland open their World Cup campaign against Romania in Bordeaux on September 9.
The committee, whose hearing started on Thursday, also gave Irish province Leinster a suspended £7,500 (8,749 euros) fine for not exercising reasonable control over their player.
Leinster's agonising 27-26 loss to La Rochelle was their second straight narrow defeat to the French side in European club rugby's showpiece match.
The committee found Sexton, who admitted misconduct, had engaged in behaviour that was "confrontational and aggressive towards, and disrespectful of the match officials".
They added he had pointed his finger at the officials and shouted something to the effect of: "It's a disgrace you guys can't get the big decisions right", with the committee also saying those words were accompanied by expletives, "most likely the f-word".
A three-man panel chaired by senior lawyer Christopher Quinlan found Sexton's conduct was "obviously unsportsmanlike and brought the sport of rugby union into disrepute".
Sexton is free to play again on August 27
The committee ruled that if Leinster commit another act of misconduct before the end of the 2023/24 EPCR season the club must pay the £7,500 fine in full.
Sexton, Leinster and EPCR all have the right to appeal the decisions, which must be done by 1500 GMT on Wednesday.
One of the outstanding rugby union players of his generation, Sexton has said he will retire after this year's World Cup in France.