I vote for 1 or 3...possibly 2. The problem with #2 is bed flex. If I understand the concept for rock sliders right, it's so you can go on a rocky trail, where you'll possibly have both front and rear flex. #2 will not allow for you to have bed movement without smacking into the slider. Now what you could do so that they extend the whole length is to make #2 into a 4 piece (2 per side) set. 1 piece for the cab, and another for the bed. As for over all design, I'd go with round tubing. The over all integrity of the slider will be uniform strength throughout the piping. Square tubing puts the strength at the corners plus a rounded surface is less likely to snag over a 90° surface. I'd also do a rounded aerodynamic design, with the tubing closest to the body being longer and the farthest piece shorter. Kind of like a bow (as in bow and arrows or bow saw), where the "strings/blade" of the bow would be closest to the body. I'll have to get pics of my friend's rock sliders on his '98 Taco TRD (ahahahahaha TURD!
) so that it might be more understandable. As for 1 or 3, I'm not an engineer, I'm going to school for biology, so I'm unsure of how much stronger/practical more cross supports would be in this case. I do know that more is stronger in certain situations, and I also know that you want the sliders to be ultra strong and support the weight of the truck so that there's little or no damage to the truck itself, just the sliders.
Just my $0.02