Arnold was sent off by referee Matthew Carley in the 10th minute of extra-time at the Tottenham Hotspur Stadium for a dangerous clear-out on Leinster prop, Cian Healy. Toulouse went on to win 31-22 to collect their sixth title.
The independent disciplinary committee agreed that Arnold "had made contact with Healy's head in a dangerous manner" but decided that the offence was at the mid-range of World Rugby's sanctions with four weeks selected as the appropriate entry point.
"Taking into account the fact that the player had previously served a suspension, the committee reduced the sanction by one week before imposing a three-week suspension," the committee said.
Arnold is free to play on June 24 but if he completes a World Rugby Coaching Intervention, he will be free to play on June 17.
In that case, Arnold will miss Saturday's regular season closer at Lyon but would be clear to play in the Top 14 semi-finals, for which Toulouse have already qualified, on June 21/22, with the final in Marseille a week later.
