LEDs are NEVER connected directly in parallel unless they are all measured then matched to have exactly the same voltage. A "3V" LED could be 2.7V or 3.3V or any voltage in between.
In parallel, an LED with the lowest forward voltage will hog most of the current and will quickly burn out. Then the remaining LEDs will also burn out the same way.
Your idea is poor to use a 12VAC transformer to make 15VDC then throw away 10V in a 5V regulator. If the regulator has an output current of 4A then it heats with 10V x 4A= 40W which needs a huge heatsink and a fan for cooling.
Instead, use the 12VAC transformer to make a rectified and filtered 15VDC and don't use a voltage regulator. Connect four 3V LEDs in series and in series with a curent-limiting resistor to make a 15V string. Then connect as many of these strings to the 15VDC that you want.