don't rule out a different reason for the rise. An induction leak should increase and synptoms increase in direct correleation with vacuum. The lower the M/P goes, the greater the leakage. It should be virtually unnoticeable at WOT and most noticable at idle, with a somewhat linear movement between them. A spread of more than 200* between cylinders is an indication that the cause is not an induction leak. If EGT goes higher than the normal peak EGT for that RPM / ignition advance setting, it is not an induction leak. Many induction leaks will cause the cylinder to get quite lean at high MP's and therefore well LOP. This would not show as abnormallhy high EGTs at low MP levels, depending upon how rich the other cylinders are.
A sticking #3 exh valve is a reasonable next investigation step, along with a borescope looking for a bent valve, chip or debris keeping the valve from fully closing.
Larry