First, please be aware that driving AI is far from perfect, which means getting them to move and then -stay on the road- is not as easy as it could be.
This gets harder the more vehicles are involved, so if you can't get it to work with the three hummers, using a single lead hummvee and a truck might also do it.
What I suggest you do is put down three EMPTY vehicles (first place down the player, then you can pick Empty from the unit placement menu), and name each of them. After that, decide who you want in which vehicle. Keep in mind, I think the hummers can only fit five people - driver, shotgun, two in the back and gunner, but I'm not entirely sure about this either: it might just be four? Then use the three commands:
moveInDriver,
moveInCargo and
moveInGunner to move the appropriate units into their appropriate places. A suggestion (even if it might not be correct when it comes to proper procedure) is to let the highest in command lead the convoy, with the second in command following, and then the third. So, for instance:
Lead: Lt
Second: Sgt
Third: Corp.
In Arma 2 (and other BIS games) convoys drive according to rank. If you don't set it up like this from the start, they will back and drive around in circles for a long while before setting off so as to drive in the correct order.
Now, simply give the squad leader a waypoint, make sure the formation is Column and Behaviour is Safe (will keep them on the road), and start. Now, since YOU are the squad leader, you have to give the order to move as well! There is however an automatic command for this in the menu: 1 (move) -> Go to next waypoint (or something like that). This will have the SL order his car to move to the next waypoint - the others will follow as they are 'in formation'. They might not follow nicely, they might swerve off the road and do other nasty things, but...yeah. Not much we can do about that >_<
Option two, which puts you in less control initially but might fix a lot of problems, is that you have a group of drivers in a -separate- group. Include the civilian truck driver in this group, and make him the group leader. Name the three drivers of the hummvees (once again in receding order of rank - note that Private can also be used, so you can have e.g. Lt-Sgt-Corp-Pvt). Now simply give the truck-driver the same waypoint, and all of the hummers will follow behind him automatically. At some point, for instance when the bomb explodes on the road, simply have the drivers of the hummers join the player's group:
[driver1, driver2, driver3] JoinSilent group Player
So then you have full control over them again - this of course means that your commandable group will be three short in the beginning.
Rather than moving everyone in, you could also have the player order the soldiers into the hummers as he sees fit. Just do the same as above, except make the truck driver's first waypoint a "Load" WP, and the player's first WP a "Get In" WP. Then synchronize (F6) them, and the truck won't go anywhere until after the whole player's group is inside the vehicles.
Can't make an example mission now, but hopefully you get the instructions somewhat!
Wolfrug out.