Why do my halogen "puck" lights keep blowing?

Only can think of a couple of reasons for this to happen. One being that after three years the socket could be going bad from all the bulb replacements and heat that was generated. The other being the brand of bulbs. If they are a bargain pack, they may be inferior. All bulbs are not equal. Check the voltage at the bad socket to see if you are getting constant power. If they seem good you could try putting them on a dimmer or get a lower wattage bulb. My guess is that there is a wiring issue at the first puck you mentioned, either at the socket or link to others on the strand.