Hi SR,
"Material Constrained Planning' is only available in SAP APO..
In ECC , you can achieve above by setting Planning time Fence for FG = 'Total Repl. Lead time' of MRP3. You also need to change MRP type to P1.
Note that 'Total Repl lead time' is longest production path(Critical path) across all BOM level & not calculated by SAP , it should be calculated outside SAP using some simulation tool & input in SAP.
Hope it helps,Jatin