From what you say, the technical handbook sounds most like a workshop manual. Parts books usually contain exploded views of components and their part numbers but no instruction. User handbooks are generally issued to drivers and have information about the vehicles controls and everyday maintenance. As you say, they are all helpful and useful to own.
It's up to you, but, if I had the choice of only one, I would choose the technical manual if I had major mechanical work to do and the user handbook if the vehicle was fairly up together. The technical manual will tell you how to strip down the gearbox and the user manual will tell you what grade of oil should be used in the gearbox. Either way, I would aim to acquire the other two as soon as possible.
User manuals are usually the cheapest to buy because they are more plentiful, and smaller, publications. Are the manuals for your vehicle available on good CD's ? All three might be more affordable if they are and you don't mind that format. It's usually manual collectors, who only hoard this stuff, that keep prices so high.