Is it normal for a 12 year old to have a 3 week period after having missed an entire period the previuous month?

Yes: During the first year or so of menses, teens often do not ovulate and set stage for the generation of the hormonal signals to trigger cycles on schedule. They can skip a month or months, and if so the estrogen stimulating her system may lead to irregular or longer flow for some cycles.After a year or two ovulation becomes more regular as do cycles for most.