I've always wondered this, how is three defender on FM ? - Seems to be very popular these days, seeing Juventus and such, I've never tried three defender though...
In FM12 I went to AC Milan in 2015 after managing Charlton they werent the best their wingers were terrible which gave me the idea to use a 442 narrow diamond (4-1-2-1-2) just like Ancelotti used with them... That formation became so successful its a preferred formation of mine now I will implement anytime I have the right players to play it...
Anyway I started to notice that in different transitions of play it often became a 3-4-3... My DM dropped back with the CBs and played pretty much as a Libero, My WB bombed forward and played like wingers/WMF and my AM pushed forward and beyond my two strikers... I used this throughout my the 5 season but it was wholely ineffective against Barca in the CL...
Now this is the point at which I decided to go with a three man backline and the trend of the 3 man backline today was another reason... I used a 3-4-1-2 (juve) and a 3-4-2-1 (napoli) the three man backline takes lots of tinkering (I would have buried the idea only I was **** bent on getting it right)... I started out by testing it on the weak teams in SA and CL eventually I started using it regularly then the latter stages of the CL came around and well I ripped Barca apart in one game but when I went away the 3 at the back had issues... Il dig up some screenshots for you but the thing about my tactic was I played possession and had a great team so its really going to be a case of experimentation but Il try give some tips on my next post...
My tactics did have some flaws I played very high up the pitch but I used a Covering defender so that took some messing with to get right and two standard CDs instead of stoppers...
If you play with wide players rather than wingbacks you need to use WMs wingers wont work... I never tried defensive wingers...
Three man backlines can be amazing but they are dangerous to use as well... I would suggest you try the 4-1-2-1-2 and try to transition your tactic to a variation of a 3-4-3 and similarly starting out using counter with a 3man backline will have more success in the short term... Thats pretty much how I did it the big thing is tinkering normal settings wont work unless you have a great group of players...
You need CBs with good positioning and pace is helpful
in WMF and CMF you need guys how are hard workers the front three can be any combination of guys you want ist the 7 players behind them that need careful selection...
One variation for a 3-4-3 you could try which I never got around to testing is:
GK
CB/SW/CB
WBR/WBL
CMR/CML
AMR/AML
CF
If you were to use that with counter you would have a Napoli style set up which is extremely solid defensively and potentially devastating on the counter but you will need a Complete Forward up top someone like Cavani himself to make it work...