Why does my 3 month old baby's head smell bad? there is a slightly yellow crust present

Cradle cap: It sounds like your baby may have cradle cap (seborrheic dermatitis). Yellow crusting could also be an infection. You may want your pediatrician to take a look and advise a treatment plan as the two plans would be pretty different. You could start with some oil and a fine comb to see if the crust comes off easily and the smell disappears. If it does problem solved.