How can a two-engine B787 create three contrails?
Contrails are created due to variances in the pressure of the air, coupled with the amount of moisture in that air.  When the 2 engines of the B787 blast their exhaust out, the high pressure of the exhaust temporarily causes the moisture to condense.  Two contrails are visible.

However, as the pressure waves from the 2 exhaust trails propagate through the open atmosphere, they interfere with each other.  This interference can also affect the pressure of the moist air between the 2 "primary" contrails, sometimes resulting in another, "secondary" contrail.  In fact, because of this, if there were enough moisture in the air, and the engines were powerful enough, there wouldn't actually be just 3 contrails.  Rather, the contrails would present themselves as the crests of a propagating wave much like the crests of water waves as a duck swims by.  In effect, we would see the 3 contrails separating, and between each of the 3 contrails, 2 more "tertiary" contrails would be visible.

Here on Earth though, the pressure variances and the density/moisture content of the air normalize far before any of that exotic pressure wave stuff could occur.  Except we DO get that one in the middle!