The fans are in reality brush-less DC motors and therefore rely on internal electronics to make them rotate.  What happens when you under drive them therefore cannot be predicted.  Some fans are rated to work down to 5 Volts but others don't say.  If you give me the model and part number I will see if I can find its' electrical rating.

These are not costly devices so why not purchase one rated for what you want to do.  3.3V are rare but if you can run at 5 Volts can find a lot of choices.

https://www.digikey.com/product-detail/en/cui-devices/CFM-5010V-052-300/102-4365-ND/7620539

Be sure to check the actual output of your ATX supply as they are designed for heavy loads on the low voltage outlets and may not regulate properly without such.

Best Regards,

Ken Harstine