Can 5th disease cause anemia in a 3yr old? has the rash now and 2weeks ago she had her 3yr checkup and bloodwork said she has anemia, is it from 5th?

Yes: This infection can trigger a pause in red cell production for a week or more in anyone. It would usually resume unnoticed and get back to normal production levels after, but since about 1% of red cells wear out daily, the pause can result in a gradual decline in the count. However, there are also many nutritional factors that could aggravate the process & I wouldn't rely on it as the only factor.