I have kept houseplants alive for up to 10 days by watering well and then putting a clear plastic bag such as a drycleaner bag over them, but never as long as three weeks. You could try wicking by placing a reservoir of water with a rag like maybe a 3 inch strip of towling going from the water to the plant pot. The problem with that is that it takes a little experimentation to find out the width of the rag required. It's easy to flood the plant. Put the pot of water at the same level as the plant or a little lower, never higher.
There are of course drip irrigation systems but you said you had no money.
I am going away for a few days as well and I bought DriWater to use. I am also switching to CFL for the duration as I am paranoid about the heat from MH since my house burned a few years ago. Entirely my fault then, but it has made me super cautious. I won't get much grow with a few CFL but it will keep them alive for a few days.
Best of luck to you.