Agreed ----Look at the other lower carburetor.----These cross over.----Looking from front of boat the bottom right carburetor feeds the bottom left cylinder.
Hook up a timing light to #1.----Take boat out into the lake.-----Run to full throttle and observe the light.-----Repeat with #2 and #3.-----Should tell you which cylinder has an ignition issue.
The engine must respond to the low speed mixture adjustment !!-----So take bottom carburetor apart.-----Do not run motor again.-----A plugged carburetor means a blockage can starve 2 cylinders of oil !
Depends on whether you do your own service work.----Sometimes parts / used parts are easier to find for Evinrude.-----Number of hours on the motor / condition also mean a lot.