Motor shield not providing enough current to 2 12V DC fans
I kinda doubt that that fan will actually draw 3A. You could probably get away with using your shield--just measure the fan current draw by directly connecting it up to a 12V supply and measuring with a multimeter. 

If it actually draws 3A, any of these sounds like a decent idea:

https://www.amazon.com/WGCD-Controller-Module-Stepper-Arduino/dp/B06X9D1PR9/ref=sr_1_4?s=electronics&amp;ie=UTF8&amp;qid=1523692716&amp;sr=1-4&amp;keywords=arduino+h+bridge

https://www.amazon.com/Diymall-Module-Stepper-Modules-Arduino/dp/B00NJOTBOK/ref=sr_1_8?s=electronics&amp;ie=UTF8&amp;qid=1523692716&amp;sr=1-8&amp;keywords=arduino+h+bridge

https://www.amazon.com/SainSmart-Stepper-Controller-Mega2560-Duemilanove/dp/B00AJGM37I/ref=sr_1_11?s=electronics&amp;ie=UTF8&amp;qid=1523692716&amp;sr=1-11&amp;keywords=arduino+h+bridge