I am curious to know how you guys have mounted your bike computer magnet to a Hed 3 Front wheel. I don’t want to just glue the magnet to the wheel, because that will certainly but a damp in the aerodynamics of the wheel. Any suggestions would be appreciated. Thanks.
Use silicone (RTV sealant from an automotive store) or even just regular glue. Put the magnet at the bottom of the cutout for the valve stem. There should be plenty of room for both it and good access to the tube for pumping. That is what I did when I rode on a borrowed pair. You couldn’t even see the magnet. Overkill – certainly! But it shows a level of OCB that this forum would be proud of…
I glued mine to the side of the valve “cave”. Got a one centimeter rare earth magnet off of EBay. The usual ones included with bike computers are girly men magnets.
I don’t have a HED 3 spoke, as I am running spinergy rev-x, however, I ended up using a crank arm magnet from cat eye. It has a stickey pad on the back and a zip tie runs through it to hold it down. It is just large enough to get a reading and has slightly rounded edges for a semi aero look.
Josh
Those are extra good because if you pull up near a rider on a steel frame, they can actually “pull” you once you get like within 3m or so. 
Guys…you are thinking way to complex. Just cut out a 1 cm x 1 cm piece of a fridge magnet and then take some black electrical tape and stick it to wherever you want on your trispoke. This does the job. All you need is a moving H field to induce an E field and then you have a pulse of current going up to your bike computer. A fridge magnet performs this trick and it is nice and flat and aero.
Sometimes, you just need a simple solution to a simple problem (of course, knowing a bit about Maxwell’s equations helps) 
Have fun.
Dev
OK, this means the fridge magnet does not have enough of a magnetic field to induce the current you need. The other option is cracking a cadence magnet open, scooping out the magnet inside the plastic casing and taping it to your disk. This is what I have on my trispoke and it works perfectly.
Dev
I use one of the Neodymium rare earth magnets on my Nimble Crosswind…they are some of the strongest magnets around. The sensor can be almost a cm away, and still pick up the magnet. You can also use them for cadence magnets…just glue them to the inside of your crankarm…very clean looking…no ugly plastic zip tie. Just use a drop of silicone adhesive (clear RTV). If you need to move them later, the silicone will just peel off. They are thin enough not to disrupt the airflow on the wheel, so don’t worry about aerodynamics.
Best thing…super cheap! $0.20 a piece!
http://www.wondermagnets.com/cgi-bin/edatcat/WMSstore.pl?user_action=detail&catalogno=0010
Neodymium (rare earth magnet) from Radio Shack. Contact cement (It comes off) and I cover it with shrink fit tape, which I hit with the hair dryer.