If you want to try to repair a keyswitch, then try the following. You can unclip the 2 parts of the housing at the sides. The top part lifts off, releasing (in order) the plunger, a spring and a little rubber dome. The contact is the black (graphite-loaded) rubber pad inside the dome and the 2 metal contacts in the bottom part of the switch. Clean the metal contacts with a cotton bud and isopropyl alcohol (propan-2-ol). You can try rubbing the rubber pad on very fine (1000 grit or finer) wet-n-dry paper. Put the switch together. Measure the resistance between the pins when it's pressed down. You should use 2 pins diagonally oposite each other for this test. A good switch is about 50-100 Ohms. A useable one is <300Ohms. More than that, and it probably won't work. If you have all the switches out, put the worse ones on the numeric pad as above. If you still have a defective switch, then try rubbing a soft pencil (I use a 6B) on the rubber pad inside. That got my worst swtich useable again. There is a special kit for this from Chemtronics which recoats the rubber contacts, but it's not cheap ($30). It could be worth using if you can't get the switch working any other way.

Putting it all back together is the reverse of dismantling it. Take great care when putting the Model 3 case back together - it's possible to break the CRT neck on the logic cage. And while it's possible to replace the CRT, new ones are not that easy to get.