Just Adding to What the Others Said
Are your EGTs pretty much in line with each other? A high CHT along with a high EGT points pretty strongly to a plugged injector.
And Terry is absolutely correct in stating that the #3 cylinder often runs hot due to the lack of an air path through cooling fins on that back side where the baffle touches the cylinder head fins. A washer on that back screw will give just a bit of breathing room for air to pass by. If you remove the plenum and look straight down, you'll see where the casting of the cylinder goes right up to the back edge of the fin, effectively stopping airflow in that small area.
If your plenum is fitting well and you don't want to create a gap with the washer trick, an alternative is to cut out a section of the back baffle, fit in a small spacer rectangular ring (0.062" thick material will work), then rivet in a back cover to seal it up. This approach gives the air a path around that blocked area down to the lower section of the fins. This is what my EAA tech counselor recommended when I didn't want to modify my plenum. Other than the hassle of removing that back baffle, it really didn't take much time. I may not be explaining it well, but you can search on this site and see pics of what people have done. It's a relatively small area that's affected; maybe less than 2" wide.
One more thing: you said you've checked the plenum for leaks and it's good. Have you checked everywhere that the baffles fit against the cylinders and the crankcase? There are several gaps made into those baffles where they fit against the engine--some are almost 1/4" wide at the cylinder flange. The inter-cylinder baffles installed by the engine supplier are also a notorious source of leaks. If you haven't already done so, get a flashlight and drop it down between 1 & 3 and between 2 & 4. If you can see light underneath that's not a fin, it needs to be sealed with RTV. You can dab RTV on the end of a dowel to get into the hard to reach areas. Again, my tech counselor had me do this. I was skeptical, but it absolutely made a difference.