Seen on a Russian MI-28 “ex helicopter”
I guess this is a line of rivets going into the end of a former, and the edges of the former are not continuous for whatever reason.
You sure these are rivets and not bullet holes? In this case I’d have an explanation.
Rivet edge distance
Rivet edge distance
??
Yes. There is a standard distance between an edge and the rivet. There is also a standard (but flexible) number of rivets. The skin is riveted onto something, but that something does not have to be continuous. There could be lots of reasons for that, equipment of some kind, other structural members and so on.
There are probably only small patches were rivets can be put. There must be 3 or them, and the edge distance need to be above a minimum. If only 2 rivets were needed, then it probably would look continuous.
My guess anyway having done lots and lots of rivets 
I’m wondering if it isn’t an artifact of the issues in forming the bulkhead to which the skin is riveted. Forming a continuous curved flange on the edge of a bulkhead involves compressing the material, which then causes the bulkhead itself to bend from being flat. Segmenting the bulkhead flange mitigates that problem, and it could be that three rivets then a break in the flange was the optimum solution here for a press formed bulkhead.
Another solution when hand forming the flange is to ‘shrink’ the material with a tool made for that purpose, or to make the bulkhead flange side of joint differently, like this, basically making one joint into two joints in a more labor intensive way:
Ah ok that must be it. Found this on google
The Russian heli must have had 3 rivets where the above has 1.
