what play is the threepenny opera based on?

John Gay's The Beggar's Opera is the source of the plot.

the threepenny opera is a comedy play, you can even say it as a dark comedy.

which is based on mackheth, mackie the knife.

It gets its name from the production on which it’s based, a ballad opera called The Beggar’s Opera, written by John Gay in 1728. In its opening lines, Gay’s opera proclaims itself to be an “opera for beggars”, the idea being that it would be so cheap that even the very poor would be able to afford it (in today’s money, threepence is worth about 1¼p).