hmm, 3 weeks is a long time. perhaps you could set up some kind of 'wick' system though, depending on how big your grow is.
in an ed rosenthal book i read about this setup.
take a kiddie paddling pool and place a wooden pallet inside it. make holes in the bottom of the plant pots and feed some thick cord through, then place them on the pallet with the cord ends dangling into the pool. finally put some water in the pool. the cord will draw water into the pots as the soil dries out.
not ideal, but cheap, easy and it may keep your plants alive at least.
take care not to damage the roots.
hope this helps.