W4ZV terrain toward Europe for 40 (   ), 45 (   ) and 50 (   ) degree azimuths: 

Using only the 45 degree azimuth, below is a plot of my takeoff angles for all 3 antennas (   ), the bottom two (   ) and the bottom only (   ).  I often find that all 3 are best for early openings (11-14 UTC), then the bottom two take over and occasionally, very late in the day, the bottom antenna alone is best.  Note that VOACAP propagation statistics (   ) indicate that 16 degrees is the maximum TOA to EU for my location.

"Figure of Merit" (right side of above plot) for each antenna combination takes into account the array's relative gain at each elevation angle propagation statistic and weights them by percentage occurrence across all angles.  All 3 in-phase has the highest "Figure of Merit" even though other combinations may exceed that for specific takeoff angles.  In general, the bottom 2 are better for angles from 8-12 degrees and the bottom alone is best for angles above 12 degrees.  Gains are overstated by 0.3 dBi since my 6 element KLM's only have ~10.7 dBi free-space gain instead of the 11.0 dBi assumed by the software.  The graphics above are from an experimental ARRL program under development by N6BV which is now available in the 20th edition of ARRL's Antenna Book.