Hi Tracecom,It would be much easier if you decide to power the clock with a battery. You still can use the 12 V supply for three LEDs in series. (A small battery for the LEDs wouldn't last long.)
You won't need a PCB because you can just use the components' leads to connect them together. The schematic below should work for any common white LEDs you choose.
Thanks to both of you for the comments.
I understand the above schematic, looks good! But i have one (dumb) question... Where does the negative part of the supply go? (or is this the gnd?).