Red itchy bumps inner elbows, behind knees and lower back. Have had for 3 weeks.

Possible eczema. please use 1% hydrocortisone cream over the counter for help. Evaluation by your physician is needed.
Try moisturizer. The distribution is suggestive of atopic dermatitis. However, it is unusual not to have asthma , hay fever or both when one has atopic dermatitis. I note that you have already tried HC cream but the OTC product may not be potent enough. Try applying moisturizer within minutes after bathing to see if this helps. If not consult a derm or allergist.