As is just being pointed out krox in a 3x2 formation is not effective as a flanking unit. It has the potential to remove ranks but if you lose one (not hard from shooting) its gone. In that case 5 krox isn't worth 275pt. I believe 4 krox in a 2x2 formation is a better option. Its 220pts (expensive, krox are not point efficient. another topic tho), and will not draw as much fire. If it does, then 4 krox is just as effective as 3 krox for their purpose (protecting flanks from light flankers, providing weight to main battle combats). Even though you don't disrupt ranks, they still give you +1 for the flank, +1 for the charge, and cause fear.
Now a 17 man skrox unit is cheaper slightly. So is that a better option? That's up to the general. My current list for 1800 uses two 2x2 krox blocks. A dual charge is mighty crushing. Do I use them in my more competitive lists? No.