My husband got a two-by-four that goes across the bed and mounted two fork mounts near the end, and for the center he got one of those short handrails used in bathroom stalls for the disabled (got it at Home Depot) and mounted the third fork mount there. That way, the handlebar of the center bike is about 4" above the handlebars of the two end bikes, thus avoiding them bumping into each other. People marvel at the ingenuity of it because it works so well but costed us maybe $20 for the whole thing. The fork mounts were given to us by a bikeshop manager friend who stopped carrying the mounts in his shop. The 2x4 was from our backyard (yes, we're one of those types who always has spare lumber) so we only had to buy that handrailing. And when we don't need the rack, we just lean it against the wall in the garage.
The only thing though is that we have a locking shell on the truck so the bikes are secured inside and we can stop for lunch on our way home or run errands. Otherwise there's really no way to lock the bikes unless they're locked to each other.