Openside flanker Williams has been banned for a fortnight and will miss the Welsh derby with the Scarlets a week on Friday, as well as the Heineken Cup trip to Castres this weekend.
Williams had being charged under the IRB's Law 10.4(a) 'A player must not strike an opponent with the knee(s)' following an incident involving Chris Paterson in the Euro opener with Edinburgh last Saturday.
The 35-year-old asserted that his actions did not warrant a red card but still pleaded guilty to the offence.
The lower end ban was three weeks but that was reduced to two after the disciplinary committee took into account factors such as Williams' previous good conduct.
Tuitupou and Fa'amatuainu were both handed three-week suspensions for incidents in the matches against London Irish and Perpignan respectively.
Munster centre Tuitupou pleaded not guilty to a charge of a dangerous tackle on Paul Hodgson during the first-half of the 23-17 loss in Reading.
He was found guilty of the offence, however, and will now miss the Magners League matches with Treviso and Ulster, in addition to Saturday's continental clash with Toulon at Thomond Park.
Fa'amatuainu will also miss two Magners League ties and the second round of the Heineken Cup following a dangerous tackle on Perpignan wing Adrien Plante during the impressive bonus-point win over the French giants.
The Samoan international will be unavailable for the league games against the Blues and Glasgow Warriors, plus Saturday's match at Leicester.
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