Yes! Your BoxPN subscription enables you to have 3 connections simultaneously.
It is possible to use the same account and setup several simultaneous OpenVPN connections, or several OpenVPN + one PPTP connection; but it's not possible to have more than one PPTP connection at the same time.
However, we would like to note that simultaneous connections might cause connection issues, so we generally advise against that. In other words, to ensure connection stability, we recommend signing up for multiple accounts if you often need secure VPN connection on more than one device at the same time.
If you have any issues while connecting your computers / devices simultaneously; please contact Support for prompt assistance.