I have attached a script to this post that will do what you want. I have put comments in the script, so you can see how I did it.
Call the script like this:
[NameOfFuelTruck, NameOfTank, NameOfGroup] exec "GetTank.sqs"
NameOfFuelTruck is, obviously, the name of the fuel truck
NameOfTank is the name of the empty tank you want the men to get into
NameOfGroup is the name of the group the men leave. To name the group, put this into the leader's init field:
NameOfGroup = group this
So, if you had:
Grp1 = group this
in the leader's init, Fuel1 as the fueltruck name, and Tank1 as the name of the tank, you would call the script like this:
[Fuel1, Tank1, Grp1] exec "GetTank.sqs"
I hope the script helps.
-Student Pilot
EDIT
Sorry, I misread part of your post, I have to adjust the script, I'll post a newer version when I finish
EDIT2
Uhg, 10:00 pm is too late for me to work coherently. I have rereread your post, and realized what you want isn't possible. The fuel truck can only carry three people, and those three people are needed for the tank. So if the three people board the tank, the fuel truck is left behind. I will repost my script in it's current version. When the script is called, p1, p2, and p3 will leave the group, get in the fuel truck, move to the tank, get into the tank, and refuel it. Once the tank is refueled, they will join the group they left. The fuel truck will get left behind.