MK126 dummy alarm

Does this kit drive a 12volt LED? Can it drive multiple (3) LEDs? Thank you

There is no such thing as a 12V led. You probably mean a led with incorporated series resistor.
Yes, it can do that, if you bridge R15 (470 ohm).
It can also drive 3 normal red leds in series. Make sure you adjust the value of R15.

Great - thank you.

As an afterthought, are there a range of values for R15 given in the instructions? Or could you please let me have a value for 3 normal red leds?

270 ohm would be a good value…

What’s the specification of the included LED?
What should the value of R15 be if I whant to use a LED with the following specification?
Voltage (maximum): 3,4V
Voltage (typical): 2,8V
Current: 20mA

Red leds: 1.7V 20mA (rule of thumb).

390 ohm would be a good value.