I was hoping to buy another 3102 myself last week and i was going to do a step by step guide to getting it working on 3CX, unfortunately those plans are on hold for now.
Try getting the FXS (the phone) port working first, then work on the FXO (PSTN) portion. Make sure you have assigned a fixed IP to the 3102. When building the FXO, put the settings in 3CX first then do the 3102. Get it to register, that's one big step. Contrary to the info in the 3102 configuration guide on the 3CX site, that says the use of the FXS AND FXO port is not supported, it actually does work. There are essentially two devices in the one box. However, problems can arise because, depending on the options, the two can interact with one another, if allowed.
Have you downloaded and read the Administration Guide? It can give you a better understanding of what the 3102 can is capable of. You won't be using all of the features so it can seem a bit overwhelming, but if you read what each setting does it can help make some sense of things . From the default settings, do a capture of each screen, then print them out. As you make changes, mark them on the printouts so you can go back and un-do something if it begins failing. There are some default settings that do need to be changed to work properly with 3CX