Does any one know what the compression is supposed to be on the three cylinder?
No, but to my knowledge Kubota doesn't make a direct injected diesel, and IDI diesels are typically in the range of about 21:1 to 23:1 compression ratio. DI engines are usually about 16:1
If you are talking compression test compression, DO NOT USE A GAS ENGINE TESTER. Special testers are used on diesels that usually go into the injector port. Also never do a "wet test" as the compression ratio is high enough to cause the oil to ignite.
Often when a cylinder has low compression it will miss at idle. Cracking the injector identifies the bad cylinder. If the miss moves around, air getting in the lines is often the culprit. Difficult starting usually is another issue.