post a screenshot
I can't tell what's a subevent, and whats another condition in the same event
but there are two ways you can do this
one that uses variables, and one that uses double families
the private variables method:
enemy pick with highest x
highX=.x
a new event
pick with lowest x
lowX=.x
then set your middle enemies x to lowX+((highX-lowX)/2)
or you could set it to lerp(lowX,highX,0.5)
the way with families would be
make the enemies members of two families
let's say low is blue, and high is red
Red pick with highest X
subevent(blue pick with lowest x)
create new enemy x at blue.x+((red.x-blue.x)/2)
or lerp(blue.x,red.x,0.5)