In all of the diagrams above, it does not matter whether your personnel dictates that you run a 3-man front (3-3 or 3-4) or a four-man front (4-3, 4-4, or 4-2-5). The gaps will be filled by different players based on the front and whether a defensive line is responsible for two-gaps or one. The defensive backs and linebackers would have essentially the same responsibility regardless of the scheme, be it a three or four-man front. Scheme is overrated! The key to playing good defense is placing the players in positions where they can be successful and teaching them the scheme. The best coaches are good teachers.
We teach not only the defensive scheme, but how the opponent will try to attack us, where the weaknesses are in our scheme. As long as the scheme is sound against the offense, that is against every play they can run, and our players understand how to react to every possible offensive action, we have a chance to be successful. If our players make the play that they are supposed to, the plays that come their way, we will have a good defense. Football, as a wise man once told me is "players making plays."