In a "lost spark" ignition system, each plug fires at the same time. In a triple, that means each plug fires three times per revolution. One of the sparks ignites the fuel, the other two are lost. If the outside cylinders are getting spark, it would make sense to check the center for spark. Could be a bad plug or a bad wire, or a bad coil. The plug should be wet if it's not firing, and the center cylinder will be cooler than the other two. It's easy to check for; switch the wires around. If the problem follows to the cylinder the wire is on, it's electrical. If the problem stays on the center, suspect carb.