The items to be addressed for the 3 year/300 hour service are listed in the owners manual. They are typically items that have traditionally been done annually on older outboard motors. It is best to have a local dealer perform the work as there are often tips and servicing that should be done depending on the environmental conditions the boat is run in, such as salt water, muddy water and shallow sandy waters. A dealer can also computer check your motor, print out a report, and look for any error codes stored in the motor's memory.
Remember that the 3 year/300 hour service is for normal recreational use in fresh water. Commercial use, high performance use, and salt water use require maintenance more often.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.