You may have a short developing between the red/black travelers in your 12/3 cable. Is there any exposed cable outside? Overtightened cable clamps and staples are also possibilities. In any event you should disconnect the feed to whichever 3-way is receiving the power (probably the indoor one) until you replace the wire. If the short is occurring outside of a junction box and your house is wired with romex cable then the short is likely up against the wood framing of your house. Sparks and dried wood aren't a good combination.