Why not simply hire someone on a 3-year contract with the understanding that the first x months will be paid for but spent in learning a new skill and the other x months will be spent in earning back the course + paid hours?
There should be a strict rule in the contract, that ending it before the 3 years are over will have a pricey consequence, so that you are incentivized to only sign up if you understand the work involved and will not quit after just doing the training.
There should be a strict rule in the contract, that ending it before the 3 years are over will have a pricey consequence, so that you are incentivized to only sign up if you understand the work involved and will not quit after just doing the training.