Hi: I have some undercabinet light fixtures from IKEA that each take 3 small 12V G4 size bulbs. These bulbs are about 3/4" long x 1/4" wide with two plug-in prongs at the end. The original bulbs were 10W Halogen bulbs but I found some nice 3W LED bulbs to use as replacements. The new bulbs work great, but the odd thing is that they will only light up if 1 of the 3 bulbs in the fixture is an old halogen type bulb. If I do 1, 2 or all 3 as LED's with no halogen bulb I just get a light flicker in all of the LED's. But as soon as I make 1 of the 3 bulbs a halogen they all light up perfectly. Any idea what's going on here? I assume it has something to do with the transformer in the fixture and it's starter not giving enough of a jolt without the halogen bulb in the string.
Halogens are incandescent and do not require a "starter". Those are for fluorescent lamps.
I am guessing the LED's do not put enough of a load on the secondary (12v) side of the circuit to initiate a steady current draw. Some power supply output circuits are electronic and not just a straight transformer. You would need a ballast resistor added to the circuit in parallel with the lamps to get all three LED's to work. But then you have current flowing through the resistor all the time, defeating the purpose of switching to LED's to save electricity.
You should get a fixture specifically designed to use LED's.
The specifics of this question are probably better answered by an electrical engineer so you would need the question moved to the electronic store section if you want an in depth response.
Your switch is broken.
Or to be more precise, it isn't a switch at all. It's a dimmer, motion sensor, light sensor, or some other kind of smart switch that requires its own power source to function. To get power, it leaks current through the bulb, which is a trick that only works with incandescent (i.e. halogen) bulbs. Around 1997 it became obvious this trick would not work with the then-emerging CFLs. So the NFPA mandated neutrals in switch loops, and the dimmer industry ignored the problem entirely for 15 years. You have one of those.
I gather this is one of Ikea's modular deals where you buy the fixtures and wire separate from the power supply/controller. I would return to Ikea and see if they can consult you on what to do, but it will probably involve a new light controller (and power supply if integral) and possibly different bulbs. You'll surely be able to keep your wiring and fixtures.