Hello, I will do my best to help answer your General Motors Vehicle questions.
The 3 vacuum line connector that runs down the right side of the transmission connects to the transfer case 4WD vacuum switch. It is located on the top of the transfer case. The switch has It has a locater pin so be sure you plug it in correctly as it will slide on 3 different ways.
3 tubes...One is a vent that is just loose at the back of the intake. One is vacuum supply to the switch the other is vacuum signal to the front drive axle actuator under the battery. Usually there will be a Y or T connection to attach the vacuum switch supply to.
This is the best diagram I can find for your application.
Do you mean the "Y" that is from the top center of the transmission? Are you asking does this connect to the fitting near the brake booster?
Again there are no vacuum lines from/to the transmission, those are only vents. The diagram I posted earlier is the only one I have available for the transfer case vacuum hoses. Its really not important where you pick up vacuum from as long as its a constant supply. The 4.3 had a few different configurations. Some would pick up vacuum at the right rear of the intake while others used a vacuum tube close to the pcv port. I don't have any other diagrams to be able to help with exact vacuum hose routing and location. The under hood label will have a general vacuum schematic for your application. I hope the label is still attached and is correct for your truck.