fix strange surface normal problem with 3-point autoleveling

the plane normal vector that was set in set_bed_level_equation_3pts was looking very wrong to me--after doing a 3-point bed probe, the z-axis seemed to move in the opposite direction that it should to compensate for bed height.

i just made the vector3 constructor call in set_bed_level_equation_3pts look like the call from the (presumably functional) set_bed_level_equation_lsq. things seem to be working for me.

i suspect that this solves Marlin issue #910 and maybe issue #1002 and some other less descriptive bug reports.