You can reinstall Corona 3, and the license will work (for any Corona version, in fact).
It would be good if you posted the issues you are having with conversion, btw. There could be solutions, or it could be something needs fixing, but would be good to know what the problems are (as we try very hard to make sure projects from previous versions will render in newer versions without problems).
Do you mean you have three times 1 WS + 3 nodes? If so, then with DR that could be 1 master machine and 11 render slaves if you also use the other workstations as render slaves in DR; or 9 slaves if you just use the render nodes (because the other 2 workstations are still being used as workstations when rendering to DR and you don't want to slow them down :) ).