Lets make sure I understand the question. Diagram #3 has constant power entering the top box and terminating on the common terminal of a 3 way switch. It goes no further than that point. Please note that the neutral does go between boxes - it should be a white wire and does not terminate on either switch.
Power is achieved in the lower (box 2?) only via the traveler wires going between switches. With the light fixture wired to the common terminal of the second switch power is delivered to the fixture only when the power can pass through the first switch to a traveler wire, then to the second switch, through the second switch to the common terminal of that second switch and on to the fixture.
Each switch box should have a 2 wire (plus ground) cable plus a 3 wire cable entering the box. The light box should have only 1 two wire cable, unless there are additional light boxes and fixtures running off of the same switch, whereupon there will be additional cables in the light box. One of the 2 wire cables in a switch box is constant power - if this is not the case then diagram #3 most likely does not apply.
At least one switch box MUST have constant power, but it is not unusual to find only one of the switch boxes with that constant power. If both switch boxes has constant power, that power passes through one box without terminating on the switch there; It is NEVER the case that BOTH 3 way switches have constant power terminated on the switch. The common terminal on one switch is constant power, while the common terminal on the other switch goes to the light fixture.