Depends on the opposition. If they play 2-3 ST or 1-2 ST and 2 AML/AMR I would go wide (or use 4 Defenders). If they play something like a narrow diamond or 4-1-3N-2, then 3 DC could do the job. Personally I don't like to use only 3 defenders, if I use 3 DC I play with DML/DMR as well...