No spark on #3

Hello maybe someone can give me some advise in this matter. I Have a Rotax 912.  the problem I am having is that the engine starting running rough, so we went through and worked on the carbs and replaced one of the carb clamps. we got everything setup and still had the same problem. We pulled the spark plugs and all looked good until I got to the plugs on #3. Both top and bottom were wet and looked as thought they were not firing. we cleaned all the plugs and checked the gap. (.025). next we cleaned the carbs, and reset everything. I cut a quarter in off the plug wire and replaced the head. We fired it up and got the same result. High trust at first gives good steady run then it starts to run rough.  Runs real rough at idle.  Checked the plugs again and same thing all look good except plugs on number 3. Both wet and not firing. The confusing part is this top on one mag and bottom on other mag. How can I get both not firing on one cylinder but the other 3 are fine top and bottom. Obviously there is something I am missing. Has anyone else ran into this and any advise.