This works:-

"c:\mongodb\bin\mongod.exe" --config "c:\mongodb\bin\master.conf" --serviceName MongoMaster3 --serviceDisplayName MongoMaster3 --serviceDescription "Mongo Instance 1 - Master" --reinstall

But this installs the service with a nonsense path:-

"\mongodb\bin\mongod.exe" --config "c:\mongodb\bin\master.conf" --serviceName MongoMaster3 --serviceDisplayName MongoMaster3 --serviceDescription "Mongo Instance 1 - Master" --reinstall

i.e. if you don't put the drive letter there it calculates the path incorrectly when it installs the Windows Service. Windows Services shows:-

"C:\MongoDB\bin\\mongodb\bin\mongod.exe" --config c:\mongodb\bin\master.conf --service
^