I would recommend having your mechanic or a mechanic doing a leak down test on the cylinders. This will give you a much clearer idea of what the problem could be. I don't believe your engine has a decompressor, so the compression numbers are likely correct. Low compression can be caused by a number of different things from stuck rings to burnt intake / exhaust valves, but a leak down test will tell you exactly which is causing your problem. Once you've figured out where the problem is you can remove the head to examine the extent of the damage. If $2,000.00 is just for the tear down that is a very steep price.
As for #3 cylinder not firing, the most likely culprits would be a bad connector, bad ignition coil or bad power pack. Check your wire connectors for any corrosion or loose contacts. You can check the coil fairly easy by just switching the coil with another from a different cylinder. If the problem moves with the coil you know it's bad. The power pack and other ignition components would require some special tools to test.
Marine Mechanic
11 years experience working on outboards, inboards, sterndrives & pwcs