In addition to the accurate information in the posts by @Critter and @rjs, also note that the short-term gains/losses screen comes up prior to the long-term capital gains screen.
If you should happen to enter a loss of $3,000 or more (e.g., $-3000) in the short-term gains/losses screen, then whatever loss you enter in the long-term gains/losses screen will have absolutely no impact.
