Curtis was charged with rough conduct and offered the ban after his tackle last Saturday left West Coast's Hamish Davis with concussion.
The action was deemed careless conduct, severe impact and high contact.
Curtis, who did not give evidence at Tuesday's tribunal hearing, argued through his counsel Justin Graham KC against both the "rough conduct" charge and "severe impact" grading.
Graham said a "low impact" grading was more appropriate, while claiming ball carrier Davis's momentum was a significant factor in the incident.
But the tribunal dismissed North Melbourne's arguments and upheld the suspension.
Simpkin later took to Instagram to vent his frustration with the decision.
The former Kangaroos skipper posted a video of Hawthorn forward Mabior Chol's tackle on Tom Stewart in last year's preliminary final as a comparison to the Curtis-Davis incident.
Chol was initially banned for three matches but had his suspension overturned at the tribunal.
"The AFL needs to get it together, the constant rule changes and the MRO is laughable," Simpkin wrote.
"Players don't know what they can and can't do anymore. Just hope for the best each week."
On Tuesday night, the AFL disagreed with North Melbourne's suggestion that Curtis had not fully pinned Davis's arms, and that the Eagles player had "free movement" in his forearms.
AFL lawyer Sam Bird said Davis could not meaningfully use his hands to reduce the impact of the tackle.
Bird said Curtis turned his body and dropped Davis rather than holding him up or releasing an arm, and argued the minimum three-match penalty for a charge of this nature was appropriate.
After hearing around 50 minutes of evidence, a tribunal of former players Paul Williams and Scott Stevens, as well as chair Jeff Gleeson KC, deliberated for almost another hour before returning their verdict.
The panel was comfortably satisfied Curtis's tackle was a dangerous one.
"Curtis came in from the side, wrapped his arms around Davis's arms and dropped and rotated Davis into the ground," Gleeson said.
"Curtis made no apparent attempt to keep his feet, to release either of Davis's arms or to rotate Davis onto himself.
"It was submitted on behalf of Curtis that it was the momentum of the tackle that caused the players to go to ground. We disagree.
"The vision shows that Davis had both feet on the ground when he handballed, and it was the technique of Curtis that result in him being brought to ground."
Gleeson added: "A reasonable player in the circumstances would've kept their feet in the tackle or at least attempted to do so."
Curtis will miss North's games against Richmond, Essendon and Port Adelaide.
The 23-year-old also unsuccessfully went to the tribunal last year to contest a three-match suspension for a tackle on Port Adelaide's Josh Sinn.
North Melbourne will decide on Wednesday whether to appeal the verdict.
GWS captain Toby Greene said Curtis was "extremely unlucky" to be suspended and may have been a "victim of his own strength".
"Everyone knows what a sling tackle is and what's dangerous," Greene told Kayo Sports.
"This one wasn't really a sling tackle but it had a lot of force behind it."
Meanwhile, Gold Coast will learn on Wednesday whether they have avoided a hefty fine after contesting Daniel Rioli's umpire contact charge.
Rioli challenged the charge via written submission to the tribunal.
If the Rioli fine is upheld, the Suns are at risk of an AFL sanction of up to $50,000, as it would be their fifth umpire contact penalty this season.
Worried by a spike in umpire contact incidents, the AFL wrote to clubs last month to remind them of the consequences for reaching the five-offence threshold.
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