I would guess a worn plate. There were millions of the 3c printed and when the plate was worn there was not always time to re-enter the plate to sharpen the design. .
I have a collection of 3c SQ's and there are a number of examples of worn plate printings that looked similar to this. It's more gradual than the missing ink examples. Here's an example where the wear is at the bottom of the stamp.
See if you agree.
