We touched on this subject recently in another topic called “primary and secondary KVA on plate iron”
http://amfone.net/Amforum/index.php?topic=31603.0.
Near the bottom of that thread I posted a file on transformer secondary RMS currents versus the DC output load current for various rectifier configurations. The pdf file is called Amveco rect notes1.
The secondary current for a transformer with full wave bridge is 1.8 times the DC load. So for the 12 VAC transformer your maximum DC load is 3 A / 1.8 or 1.67 A DC.
The secondary current for a transformer with full wave center-tap is 1.3 times the DC load. So for the 24 VAC center-tapped transformer your maximum DC load is 3 A / 1.3 or 2.3 A DC.
An interesting experiment would be for you to tack together each circuit with a 1.67 A load and measure/compare the primary ac currents.