Welcome aboard alex.
Basically theres main 3 things that could be the problem.
1) low hydraulic fluid. when the fluid is low it'll start compressing air in the lift cylinder which won't hold it up very long. Check your fluid.
2) the 3ph lift piston and/or it's rings/seals are worn allowing fluid to seep past which relieves the pressure. Remove the the 3ph piston assembly repair/replace as necessary.
3) the 3ph valve's seals are worn allow fluid to seep past which will relieve the pressure. rebuild/repair as necessary.
These aren't as common as the 1st 3 listed above but It could also be a weak hydraulic pump ( a pressure test will be necessary to trouble shoot it), one of the external hydraulic lines is sucking air (check any lines for wear etc.,) or if the tractor has one, the lift speed control is defective and/or closed ot blocked.
2 & 3 aren't extremely complicated but can be if one isn't experienced in hydraulic component repair. www.messicks.com (one of our sponsors) used to provide an on-line parts manual and ordering service and www.tractorsmart.com provides a free parts manual download and also has on-line ordering service. Both have 800 #'s and may be able to give you alittle help. It wouldn't hurt to visit either.
I haven't run across any "free" WSM's (work shop manual) but they provide detailed repair instructions and are what the pro's use. Unless someone happens along with one on PDF that they'll share you'll have to purchase one from your local Kubota dealer (who may also provide you some tech help) or from one of the sources I listed above.
Good luck and let us know how it goes in your repair.