Hi Michael:
I recently had a problem with one of my BeoLab 3's and it too was in the power supply board.
I, unlike you, did not get a yellow LED, as the fault blew out the fuse and the speaker was completely dead. I took the unit apart (very easy) and first tested the fuse and it was open (blown). Further sniffing and viewing showed it to be the transformer.
I called down to Atlantic Systems in Hanover, Massachusetts (USA) and they explained to me that, fortunately, there is in place an exchange program whereby you can turn in the entire internals - which is referred to as the chassis - in exchange for a rebuilt one. I ended up paying approximately $300.00 USD plus shipping as opposed to approximately $1,200.00 for a new one. I installed it myself and it now works perfectly. This is a very "black-box" approach as the chassis assembly includes everything except the driver, passives and case. It is all self contained and attached to the mounting frame. Also, no need to remove any of the many cables from the boards themselves as the replacement will come with them attached and they want the defective one to have them left on there. Take photos as you go to ensure that you run your wires in exactly the same paths as things are really tightly packed in there.
If you are at all adept with a torx driver, you can service the speaker yourself. I would start by downloading the BeoLab 3 Service Manual here on BeoWorld as it has some great drawings that will guide you along. When you pull the chassis, do remember to also remove the very small LED board as they require that too. I had not known this and had to ship that down to them separately.
If you decide to go ahead and replace the chassis yourself, do be careful on reassembly in that you do not strip any of the screw insertion points in the case. The screws are steel and the case is aluminum. Tighten just enough to do the job. I learned this lesson with plastic parts and use a very light touch when torquing down screws. Also, lay out a soft towel so you don't mar the soft rubberized color coat as you move forward with disassembly.
Should you have any other questions, please feel free to ask.
Michael