What do you mean? From seed planting it, walking away for 3 weeks, then coming back? is that what you mean?
Or sprouting then leaving it?
Indoor i dont think so, outdoor diff story
Either way three weeks of not watering, no medium out there will retain that much water, if you have a hydro system that is plug and play and you just set the timers, diff story
What you working with, set up etc or if your outdoors what climate etc