Hello, Flat foot in 3 yr old kids is normal. As the child develops and grows the arches of the foot assume a more natural symmetry providing the normal arch of foot. Your paediatrician is right in advising you that u have to observe for upto 5 yrs of childs age . Although if possible u can definately post a picture of ur childs feet or rather consult a paediatric orthopaedician for a examination. No need to worry its normal at the kids age.