High voltage three phase transmission lines
Actually there are many different types of high-voltage transmission line towers in North America, although that is certainly one type that's not too unusual.  The really high voltage lines are held with different towers.

I can't even guess where you got the idea that tower is carrying six 3-phase lines.  It is clearly carrying two 3-phase lines, one triplet on each side.  There is also a lightning catching wire on top.

The individual conductors are made of multiple wires in this case.  This has two advantages.  First and obvious is that the current carrying-capability scales with the number of wires.  Second, from the point of view of the E field a little distance from the bundle of wires, the bundle looks like a single large wire.  This large diameter bundle is less "sharp" to the E field, which helps reduce corona losses.  Sometimes you see the bundle deliberately arranged in a circle when the voltage is really high.

Second picture

You have now added a second picture of multiple towers.  Here you can see each line of towers carrying one 3-phase transmission line with two lightning catching wires above.  If you look closely at the full-res picture, you can see that each phase is made of three wires, and this time they are deliberately held in a circle by spacers at regular intervals.