How does three phase add up?
The easiest way to think of three phase is as three "hot" wires and one neutral wire. Between any hot wire and neutral you will measure (say) 230 volts AC and usually a single household will receive one hot wire and neutral. Several houses further down the street a house might receive one of the other hot wires and neutral. 

There are three circuits sharing a common wire called neutral and, as far as any single phase household is concerned they "see" a hot (live) and a neutral. Different names in different countries sometimes of course.

You could mistakingly supply two hot wires to a house and the voltage wouldn't be 230 V AC anymore - it would be about 400 volts because "line-to-line" voltage is \$\sqrt3\$ times bigger than line to neutral (often called "phase voltage"): -



So if Va is 230 volts and Vb is 230 volts, Vab is about 400 volts. It's just geometry.