It very well could be absolutely nothing but, any abnormal discharge
or bleeding no matter how slight, after menopause
is deemed to need checking just to be on the safe side.
Three years with no periods is definitely post menopausal but, that doesn't mean that our hormones have completely settled down and can still have a surge that can create some spotting temporarily and be perfectly normal. However, again, only a doctor can determine this and best to be safe than sorry.