P.W.
Has she been to a doctor? If you are 100% sure it is hives, then hives can go on this long. She could even have a virus that causes them. The different shapes you describe are throwing me off a bit though. She could be getting bites that are causing hives, so check her bed, etc... If it is hives then a benedryl should make them go away...at least temporarily. I don't know how much a 3 year old can take, so do your homework. The "skinny" ones described made me think of poison ivy that tends to go in lines. With children I always like to err on the side of caution however. Two weeks and a three year old.....if you haven't already I'd see a doctor.