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Suppose you have the situation (R = Roach, W = Wraithwing, B = Beethro, . = floor, # = water)

.###....
........
RW##..B.
.###....

Move E (6). B backs away, W follows him by one square over the water, both are still on the same row.

.###....
........
R#W#...B
.###....

Move SW (1). B approaches to a four-square distance, but gets off W's row, and is now slightly SW of it: W flees NW to regain the 5 square distance.

.###....
.W......
.###....
R###..B.

Now move N twice (8). Because B maintains the five-square distance from the W, the W does not move, and B can get on its column. The W sits in the path and blocks the R.

.###....
RW....B.
.###....
.###....

And then move N again.

This works on all three roaches, with the position and directions rotated by 90 or 180 degrees - the W is in place *before and after* the roach enters the desired column, and does not move, so movement order does not matter.