Hi,
As you correctly wrote cisco 7931 model is not supported by 3CX, so we can't guarantee that everything will work fine, but you can try it on your side.
To test the provisioning and functionality you need to do the following steps:
1. Sure that phone using FW for SIP and not SCCP;
2. follow the steps described here: http://www.3cx.com/sip-phones/cisco-7940g-7960g/ and just during the provisioning choose cisco 7941. The config file will be generated and you will be able to see is it possible to provision the 7931 with existed provisioning template.
Hope it will help you.