Breastfed 3 month old has eczema, gas and occasional diaper rash. No dairy has helped but he still has episodes. What else should I eliminate?

Family Hx? Many times eczema is a form of hereditary dry skin in an infant with a family history of allergies etc, so look at family history of allergies, asthma and /or itchy dry skin. Also do not overlook the environment. Are the pollens in the air he is exposed to? What are you using to launder clothing and bed linens? What are you bathing with, and what lotions do you use?
Eczema. You have to be very careful about using elimination diets in children of this age. Breast milk is the best nutrition he can get. At this point, since eczema at this age is often transient, may be to use the otc cortisone cream for the rash and wait it out a few more weeks. If it is still a problem, see a physician.