It doesn't matter where they are stopped, within 10 minutes of a stop that lasts that long they must be placed 10 feet aft, (assuming you're on the right side as you're facing) 100 feet aft, and 100 feet afore. That means they're walking 10 feet aft and placing one, 90 more feet aft and placing another, 200 feet forwards so they're placing one 100 feet ahead, and then 100 feet back to their rig. I am admittedly not great at math, but this seems to me to add up to 300 feet, unless you've got two humans in the truck willing and able to place warning devices. In that case, each person only has to walk 200 feet, and I won't be so petty as to count both of their steps since nobody has to walk the full distance.
With that out of the way, let's have a more interesting discussion. There's no reason why a vehicle cannot have self-deploying reflective warning triangles based on radio controlled vehicles. It should be doable with vehicles wholesaling around $100. I think it would be most prudent to use tracked vehicles. It would probably cost more for something to pick them up off the ground again than for all three. And come to think of it, better to have four just in case. Alternately, how about a flare mortar? Wink wink for the many humor impaired out there.