QCAD 3 is a new, separate application that can live side by side with QCAD 2.
If you use the Windows installer, please make sure that you don't install QCAD 3 in the same directory where you have previously installed QCAD 2.
If you are not sure, download the ZIP file instead and extract it somewhere on your hard drive. This allows you to run QCAD 3 without installing the software.
With the beta version, the installer did as you suggest and I could run both versions together. The RC1 installer detects QCAD 2.2 and does not give me an option to make another directory.
I will try the ZIP.
Thanks