U/A has shown trace blood in urine 3 times. Ultrasound & CT scan normal. Bloodwork also. What could this be? What should I do next?

Don't worry too much. Less than 4 red blood cells per high power field is normal.I suspect your test showed more than this as you had an ultrasound and CT done.**Your physician may want you to have a cystoscopy to look into your bladder,however bladder cancer would be very unlikely due to your age and lack of gross(visible)blood in your urine.If renal function is normal and no proteinuria would recheck U/A yearly.
Trace blood in urine. Sometimes trace blood is an indication of beginning of UTI. Simply increase your water intake to at least 64 oz of water a day and then in 2-3 weeks have a repeat UA done.
See urologist . Need to know more about your profile, age, gender, medical hx. With persistent even recurrent evidence of blood in urine, a cystoscopic evaluation (to inspect the bladder) can be necessary.