Narrow parallel bar dips, ie, shoulder width, done so that your elbows at at a 90 degree angle will mainly target tris, done with a wider grip and brought down deeper will hit chest.
Three inch anything is a waste of time.
My gym is full of those guys thaashit described, throwin up huge weights for 1/4 reps, never gettin any bigger, and laughed at by anyone who knows what they are doing.