Three weeks does seem early for any hairs to start drying up but it does happen and sometimes you never figure out the cause. Other than the excess heat or being too close to the light the most common causes are from excess exposure to direct air flow, repeated friction against other parts of plants or other objects (usually also caused by too much direct air flow), accidental pollination (usually herm), or damage to the supporting stem causing the relevant buds to ripen prematurely.