You will need to create some additional TAP-WIN32 adapters if you haven't already.
If you are using OpenVPN 2.3.x or later, run addtap.bat:
If you are using an older version of OpenVPN, run the tapinstall command
C:\Program Files (x86)\OpenVPN\bin\tapinstall.exe
(NOTE: Maybe you should open the cmd with Administrator Privileges)
Obviously you will also need to make sure that nothing about your various VPNs conflict with each other. For example if one is modifying the default gateway you are probably going to have problems. If nothing is changing the default gateway and there are no overlapping IP addresses then you may be ok.
I am not certain if it is needed by I also renamed all my TAP-WIN32 adapters with names like
VPNDEV3. In my openvpn configurations I specified the device I wanted to use by using the configuration directive