I just experienced recently that up to level 5, the 3 DC formation is going to get killed by team that are around your strength. All is well thru level 1-4. Higher than tht, still works against lesser teams but not teams stronger or just slightly below your average (yes, 5-8 below as well). Switch to a 4-man defence and see wht happens

Other than tht it is just a long overdue slump or the players u brought in this season just do not fit tht formation.