Can you save an Autoflower that is too small for it's age?

If it's this size at 3 weeks old, there's not much point to continuing with it... Autoflowers have a pre-determined schedule and they are usually going into pre-flower about now... this little lady is nowhere near ready for that. For your next grow, you need to be giving your plant at least, AT LEAST, 12 hours of light a day... autos will grow with that amount of light but they won't perform to the best of their ability... ideally, you want to have 18-20 hours of light every day. I would consider using a tent indoors with at least a decent lighting set up - you don't have to get terribly fancy about ventilation and all that if you're not worried about the smell they produce. But for this little one, I think her time is done. Sorry!