Good chance the tiller is too wide for the truck bed. If so, then it would have to go lengthwise. In such a situation, the tiller couldn't be put on the 3ph and put in the back of the truck. The tiller also couldn't be put in over the side of the bed with a 3ph.
If you lift it with a chain, remember that loads tend to swing and twist. Loads can easily bash into a tractor if lifting from a loader bucket. If a load is wide, a loader may not be able to lift it very high before the load runs into the tractor's grill. It's also a good thing to keep in mind that loaders and 3ph's lift in arcs--not true verticals. There have been more than a few loads pulled off docks and walls broken by forgetting that loaders & 3ph's don't go straight up and down.
What all that means is that I had quite a time getting a box scraper out of my utility trailer (It was sitting on top of 3ph pallet forks and couldn't be pushed out the back). I lifted it from the side of the trailer with loader. The scraper first tried to bash the far trailer wall, and then tried to bash the tractor grill when it was lifted high enough to clear the trailer. The scraper couldn't be lifted lengthwise since it would come back at the grill like a battering ram as soon as it lifted. Fortunately, my fairly strong brother-in-law was there to steady the scrapper (may not have been too safe)
Of course, I don't have these type lifting problems now that I have the 3ph pallet forks, but that probably doesn't do you much good if there's not even a loader. A counter-weighted boom, mentioned before, is a standard farm device, and they pre-date tractors. You don't even need a 3ph. Trailer hitch hoists are available from places like Northern Tool, but I don't know if such a thing would actually work for swinging a tiller into a truck bed. Many people around here have loading docks, which are simply truck-bed height verticals dug into the side of a hill. Some get fancy with log retaining walls. If you had a dock and the tiller was on dollies, it could just be rolled into the bed.