Classic high speed detonation, its melted like a mars bar in a furnace.

Could be anything really, you will likely never 100% know for certain, but having teh injectors labelled and flow tested is a start as No:3 is significantly cleaner than the others suggesting a lack of fuel in comparison, although clenliness in itself means nothing as an engine running 11.5:1 max on the power will actually stay just as clean as that pot at all times if it has closed loop.