Roles for 3 man defend

I'm currently playing a 3-5-2 formation with wingbacks and three central defenders and i'm wondering what roles are best for my central defenders.

My formation is:
GK
CD-CD-CD
WB WB
DLP-BWM
AM
P-T

I've tried to use the two side-centraldefenders to "defend", and the man in the middle with "stopper" as instruction so he can push up in the spaces behind my midfield, but I struggle to keep a clean sheet.

Do anyone have any advices?

I also have this style of formation. But have found that whichever role i had gave my CB's, the result is still the same, they finish the game on about 65% condition. They become exhausted by half time...... Then goals go in just before the break.

I have tweaked my style of gameplay and i am achieving better results then the first part of the season.
I am fourth in the Championship with Sheff Wed, and my last 9 games have all been wins, 11 games to go, and i think promotion could be gained. but i am not counting my chickens yet.
I play controlling style, short passing, quick tempo,(i have pace up front) i play slightly higher up the park, and always closing down, i don't give them any space, and my possession is always over 60%. Even when i lose. Sometimes i will start a game with overload, esp when playing a better team or if we are underdogs. Once we have about 25mins on the clock i revert to controlling the game. And i have had some very good results, FA Cup against Sheff Utd away from home, UTD were favs and after 6mins, we were 2-0 up. They never got going and the scoreline was VERY pleasing, finished the game 5-0. I even had Semedo sent off and still controlled the game even with a player less.

My Keeper is set to let defender collect the ball, and although i am only averaging 1.3 goals a game, and since tightening my back line i have dropped to conceding .6 a game. Better then first 10 games, 2 goals a game and scoring only .25 a game. Glued to the bottom of the table with only 1 point from a possible 30.
Operation Uphill struggle commenced.
Its good that my team are used to my tactics now, means i can concentrate on bringing/scouting players for next season.
I know who will be leaving, but not sure who i want coming in to replace them......