You're correct - vent hose is the 3rd line. Here's a diagram
. Where it says "Vent to atmosphere", that is a line that goes up under the hood and is attached up near the firewall on the passenger side (if I remember correctly).
The other two lines, you are correct, one goes to the 4WD actuator and the other one connects to a "tee" near the vacuum ball (does not go directly to it, though).