LED circuit parallel
Your LEDs are drawing too much current for the adapter to handle.

When you put 3 LEDs in parallel, the total current is the sum of the three currents. So in your case it adds up to up to 3150&nbsp;mA at 12&nbsp;V.

Your power supply is only rated to 1500 mA, so you are trying to draw twice as much current as specified.

You need a beefier power supply to make this work safely. Add in the fans you are powering and you might need something like 3.5&nbsp;A of current capability.

For 5 LED, it would turn into 6&nbsp;A.

As you did this wrong, I'm wondering if you are providing a means to limit the current for each LED, you might overload them too if you are just connecting them to the 12&nbsp;V.