im not good at this game, but i seem to run a relatively solid 3-4 defense, and figured i could share some insight. what i have to offer isnt fool-proof, or earth-shattering, but it works for me and i think it can work for you.
1. personell for the 3-4
the first thing you would want would be atleast 3, but preferably 4 good LBs (san diego, bengals, steelers, bears, etc). i run a 3-4 regardless of what team im using (with the dolphins all the time, who have 1 maybe 2 solid LBs), but that would be ideal. a big fat DT to clog the middle would be perfect. at DE, i would either want REALLY fast ends, or really strong ends. for the most part, your DEs wont be the ones creating pressure on the QB (unless they are really fast), so it would be nice if they could hold up the person trying to block them, or even push the blocker into the backfield. either works fine, but really fast DEs give you more flexability. safties who can tackle, and with some speed, are an asset. most of this seems self-explanatory, but i figured id mention it anyway.
2. my set up
my base set up, from every 3-4 formation (normal, over, under, solid), when the run is a threat, is to spread the Dline (L1 + up on the Dpad), loop the Dline (L1 + up on the right analog stick), and pinch the LBs (R1 + down on the Dpad). no matter what play i call, when the run is a threat, i set my defese up like this. doing this seems to hide the LBs a little better, allowing them to either shoot gaps blowing up the run, or simply just clogging the hell out of the middle, leaving the RB no room to run. tackling with the saftey is a must on alot of plays, as this set up will usually allow you to run free and make plays. against passing downs (2nd and long, or 3rd and not short), i usually just spread and loop my line, but i wont pinch. by not shifting my line, it isnt obvious where the pressure is going to come from. more than anything else i can tell you, i think this setup is the most important. most guys say they have a problem stopping the run from the 3-4, overall i dont with this setup. i will conceede that the 3-4 defense is not going to be very successful trying to stop 3rd and inches type plays. the 3-4 D will give up yards on the run, you just hope its 2 here, 3 there, etc.
3. the formations (normal, under, over, solid)
i use all of them. normal has the most amount of plays in it, but all are nice, and if just going by stock plays, all have some different ones. i use the 3-4 book.
anyway, the only real formation i use for a specific purpose is the 3-4 under. this is my stretch run killer (from strong i, or i normal). pick 2 man under (and do my normal setup of spread/loop line, pinch LBs), bump each WR, move the SS a little to the right of your screen (a little outside the TE), and control the FS. this setup will either allow the MLB to shoot the gap and crush the RB in the backfield or shoot the gap and force the RB outside way too fast. and almost everytime, the FS you are controlling will go unblocked, allowing you to run to the right of your screen to make the tackle for a short gain or even a loss. i used to get killed by guys who made a living off this run play, cuz using 3-4 normal gets the FS you are controlling blocked, instead of freeing him up. once i figured this out, i was able to hold guys in check who abused and overused this play.
4. my philosophy
blitz atleast 1 LB on most plays, try to mix things up, and bend but dont break. ill take small chances to get sacks, but i usually set up coverage behind it to avoid giving up huge chunks of yards. it sounds basic, but it works.
5. playing man D
when playing man D, i like to choose the double X/Z/TE plays as my base D. these have an OLB blitzing (i know the 3-4 normal has one where the OLB is on a delayed blitz, thats cool), one saftey in a deep zone, and the other doubling a WR or TE. what i tend to do when using these plays is to shift the coverage of the saftey already in a deep zone over to his side. so if the FS is in a deep zone in the middle of the field, ill press triangle + left on the R analog stick, to shift his coverage onto his side of the field. then ill take the SS, who is doubling the WR or TE, and put him in a deep zone. **NOTE** you HAVE to control the saftey you put into the deep zone. he will not cover the deep ball very well if you dont. its like he lets the WR run right by, even though he shows he is in a deep zone. let the CPU control the saftey you shifted over, and you control the one you put into a deep zone. just so your opponent cant key on your blitz side based on who you control, leave the saftey doubling the WR every now and then (be sure to freeze the D when coming out of the huddle, so the saftey doesnt run over there, or click on him real quick, move him a step, then click off). i just keep switching blitz sides. alot of the time, if the offense doesnt have RB blocking to that side, or a TE blocking on that side, the blitzing LB gets to the QB fairly quickly. i dont claim that this will create insane heat, but its usually enough to force throws, and you still have great coverage over top.
playing 3-4 2 man under: this has MLB #2 in a hook zone. you can either blitz him, use him to double someone, or put him into a zone where the offense keeps throwing. i like to flip the play, and put him into a purple zone, to keep the TE corner routes on lock.
other man blitzing plays i love: OLB dogs fire (both OLBs blitzing), MLB crossfire and storm blitz (both MLBs blitzing), trio whip man (ILB and OLB of the same side blitzing around the outside), and strong and weak blitz (ILB blitzing middle, OLB blitzing outside). all of these are cover 1 blitzes. to protect yourself deep, and to still cover everyone, put your best/fastest CB on their worst WR and dont bump. if your CBs suck, or their WRs are all studs, this can be tricky. otherwise, and especially in longer downs, just put the other saftey in a deep zone (shift the other one over first, then put that saftey in a deep zone and control him). sure, you give up man coverage on a RB/FB/TE, but they arent the biggest threats at that time. mixing in these plays, especially after youve played man D with 2 safties over the top, will usually suprise your opponent. dont abuse these cover 1 plays, cuz a good player will kill you if you do. again, none of these are nanos, and all can be picked up with the right blocking scheme, the idea here is that if you mix this in with 2 man under, your opponent will usually just keep a RB/FB in to block on whatever side they thing the LB is blitzing from. if you bring 2 LBs, you can usually get more pressure on the QB than they though was coming.