Outdoors and full sun would be the most sensible option.
Leaving a hydro system running for 3 weeks without daily monitoring would be a dangerous situation, not for your plants, but the risk of electrical problems could result in a fire or other drastic threats to your residence, like flooding.
Too many chances for something to go wrong with a hydro system to be left unattended for 21 days. Danger! Danger! Will Robinson!
Hope this helps,...........
Organoman.