Hard to say without looking at the firearm, but you *have* given enough information to get the good suggestions above.
With hammer-fired guns like this, there's also a possibility that the slide is perhaps 1/16-inch back from being fully forward. No safety problem with the SIG if the hammer falls, because the barrel/slide are together and fully locked for something like 3/16 of an inch before unlocking begins.
The problem arises from the "shank" of the hammer contacting the bottom rear corner of the slide on the way down to the firing pin. Sometimes, this rams the slide the rest of the way forward with enough energy left to fire the round. Sometimes there's a light strike.
I suspect this because of the occasional *3rd* hammer fall firings you report. Long cases, high primers, goobered cases, dirty chambers, burrs or sticky spots on the slide/barrel or barrel/takedown lever/camming block, and various slide/frame fit or lube problems can cause this.
You may want to videotape your next shooting session and carefully examine the frames immediately before the next jam. Putting a thin white line across the frame/slide will make it easier to see if the slide was not fully forward just before the offending shot.