Best place to take off vacuum for distillation
Assuming that you are using a typical distillation set up (boiling container, fractionating column, distillation head, condensation tube, collecting adapter, and collecting vessel), and the the "three-way" you are talking about is the distillation head. It is much better to put the vacuum take off adapter right before your collecting vessel. The reason is this set up creates a vapor flow that goes through your condensation tube, and most of your fractions will be condensed and collected.

If you attach the vacuum take off adapter on top of your distillation head, most of your vapor will take shortcut to your pump through your adapter instead of going to the condensation tube and the collecting vessel (this direction is a dead end). 

Actually the top of the distillation head is the place to put your thermometer to measure the temperature of vapor, not for anything else.