The 3rd block is typically used as a splitter, allowing you to configure it inline with your air tubing (and then having either a 1/4" or 3/8" female port coming off at 90 degrees.). Sometimes this is used to attach a leader hose to a hose reel, or you can also just attach a 1/4" Male NPT Quick Coupler directly into the block as well which is also common. If you want to use the block as an additional air drop, you can also purchase additional accessories to complete this. Thanks, Primefit
The tube is 1/2 inch outer diameter and 3/8 inner diameter. the nylon tubing is only 5/64 thick
You'll need something to seal the threads. Either teflon tape or a bit of pipe dope will do. Different retailers seem to sell Primefit kits with different sets of fittings.