When in Cover 3, you’ll have one or two line backers covering the middle (the yellow zone icons). If there’s one, I play as him and line up more towards the side with the slot receiver. If there are two LBs covering the middle, I play as the one closest to the slot receiver.
When I need to blitz, I’ll usually play Cover 1 (one safety covers the deep middle, the other is in man coverage or covering the short middle). If the MLB is in zone or QB spy, I’ll play as him. If not, I’ll switch to the safety covering the short middle.
TL;DR - the defender covering the short middle, usually the MLB.