This will be a long post and I apologies for the scattered nature of it; I am typing things as they come to mind.
I generally avoid cover 3 on third down just because it has too many holes and the AI likes calling plays that are designed to beat it. That reminds me, make sure you have the option to show past plays on and pay attention to it.
On 3rd downs I tend to run either 2 man, cover 4, or a blitz.
Be careful with zone blitzes, as they have weaknesses and on 3rd down the QB will find them. If I blitz on 3rd down I usually blitz 6. No point doing otherwise imo. I like the following blitzes:
nickel double A gap mid blitz
Nickel over storm brave
Nickel overload blitz
Nickel wide 9 odd lb dogs
Those force the offense to keep a back and/or TE in to pass block. If they don't you will have a free rusher. In my experience 5 man rushes are useless vs the pass.
My go to blitz is LB dogs out of wide 9. I only use it on 3rd downs, and love using it vs teams that like splitting out the TE (can't pass block so you will get instant pressure). I typically do the following:
Man align check, make sure there are no problems (e.g. a safety running across the formation in order to pick up his man)
Look for pass, always do this on passing situations.
Play over the top, Its a zero blitz so this is needed
Check for trickery from the offensee.g. #1 WR in the slot. IF present consider checking into something else
Press Jimmy smith He has 95 press, why wouldn't I?
Shade where needed E.g. If it is 3x1 shade the isolated WR inside,
User one of the DEs Usually Dumervil, he will be 1v1 on the edge.
Be careful doing things like shifting linemen/linebackers when you blitz. The plays are designed to attack a certain gap, if you move them without reblitzing it creates problems (e.g. over storm brave, shifting LBs to the left leads to the Will LB running into your DE)
Make sure that whatever you do is done for a reason. Never "sometimes press, sometimes don't" just to change it up. You should only press if the advantage is there. In my Ravens CFM I press with Jimmy Smith 90% of the time. The hell with being predictable, I'm going to play to my strengths. Conversely I never press with Webb or Williams because they are not good at it.
So make sure you use the coach stick to check matchups. If the WR has the edge in route running be very careful. Check the pass blocking matchups as well. Attack where it is red. If you user the linemen this should determine who you control. To determine whether or not to press, use the coach stick. My general rule is if rls is -4, I'm pressing.
I don't know about your team, but sometimes you may just need to accept not having great personnel. I cannot stress using coach stick enough.
In my opinion there is no master strategy to be used all the time. Almost everything I do on defense is due to matchups and what is happening in that game.
Zero blitz = Control a edge rusher
Zone = Control a underneath defender and read your keys.
Consider changing any flat defender to a curl/flat zone.
Playing against teams like the Bengals and Lions is a pain in the *** because they love moving Johnson and Green around. It sucks to realize far too late that Megatron is matched up vs your SS with no help because you are blitzing.
It should not take long to learn where any problems are. If you are able to this is where you user a specific defender (TE or slot receiver killing you? user a zone defender lined up over him) Crossing routes got you down? Control the hook defender on the opposite side of the WR running the cross (don't try to chase them down, just sit at the end point and meet him there)
When you user a zone defender be disciplined! Don't turbo all around the field. Read your keys, get to your landmark, and keep everything in front of you. We all like user picks but that should not be your goal.
Most importantly, use coach stick! Get in the habit of checking this every play.