My 3 yo has dk yellow/orange urine. doesn't drink much. when he does it gets lighter. since drinking more fixes it mean it's not kidney or liver prob?

Dark urine: Your child's urine being yellow/orange in color when your child does not drink much is consistent with your child's kidneys excreting a concentrated urine. You found the answer yourself when you stated that when he does drink, the urine get lighter. There is no evidence that your child has a liver or kidney problem from your description. To be absolutely sure, get a urine analysis and blood work.

Normal variation: Urine is one form of waste disposal and the color intensity will vary with the dilution of the urine. A habit of low fluid consumption will lead to darker yellow-orange urine while high fluid intake can make it lose any color.