Cool, Ill give that a shot. I dont have any means of measuring my CHTs so I dont know.

Induction leaks are worst at idle when the difference between barometric pressure and manifold pressure is greatest. If your idle is not substantially affected you are not likely to have an intake leak.

check slick sb1-15a if you have slick mags

There's more to carbs than just carbs...

My RV-9A had really high EGT on one cylinder when I got it. I called Marvel-Schebler to find out what to do about it and asked about enlarging the jet. NO! NO! or something like that...

I traded my carb for a MA-4SPA 10-3678-32. The significant part of the number is that part past "MA-4SPA 10," which by itself is an incomplete specification. The rest of the number, in my case, meant that my carb ran richer than other models. And, mine was bench calibrated at the rich end of the scale.

Call 'em up (if that's who made your carb.) They gave me excellent tech support and kept me from wasting time and screwing things up.

Sounds like ignition issue. When one plug stops firing in a cyl, the timing is retarded, which causes EGT to rise and CHT to drop. It will also create a small to moderate performance loss and slight roughness. The mag drop tells the story. When you drop the mag that is not driving the bad plug/wire, you will be running on three cylinders and she gets rough. Dropping the other mag will be noticably less rough, as it is running on 4 cyl. Watch the EGT during the mag drop to see if you have a bad wire/plug. EGT falls like a rock if there is no spark in a cyl for the single, selected mag. Don't be afraid to do mag drops at cruise power. Some ignition issues (bad coils especially) will not appear at lower power settings like a runup.

Each engine is different of course, but 1600 seem a bit high for a mixture issue. If you see an EGT that is higher than the "known" peak EGT for that cylinder, the cause is not mixture related.

Larry

Check for accuracy of EGT Sensors
 

I recall you should read 1000F with a candle heating the sensor.

Check plug wire
 

If the EGT in #3 rises faster than the other EGT's it can indicate ignition issues. Possibly only one plug firing and excess fuel is burning in exhaust near the sensor raising EGT temp and this condition causes rough engine. You mentioned checking plugs and mags, but not plug wires(unless I missed it). Could have bad plug wire.

The plug wires where tested and checked fine. I did an inflight lean mag check at cruise power yesterday. Had a drop of 100 and 110 rpm and minor roughness. I also ran it with carb heat and seen an increase in temps on 3 cylinders but that hot #3 drastically declined in temp. About 70 degrees

When finding peak egt. #2 cylinder peaked at only 1350 degrees. Which seems strange to me and #3 peaked very early @1620 and actually fell 150 degrees when finding all other cylinders peak

Hot egt
 

Maybe this was covered already and I missed it But I would check those spark plugs- remove them; clean them; gap them make sure that number three spark plug resistance is within range and not broken or cracked.
Also redo the mag synch make sure it?s at the 25 or whatever the 320 is for TDC and both hit at the same time.
Calling carb TS is a great suggestion, they will know how your carb was set and what to do to get it corrected.
I feel your pain, for years I fought CHT issues but in time, I got it resolved, and so will you.
Best of success with your troubleshooting

Exhaust Valve?
 

Has the exhaust valve on #3 been checked to see if it is sticking or not seating?