Hi OP.
Firstly, try not to stress too much about it.
When you go to the GP, be factual. Tell them you've been ttc for over 3 years with your partner but not gotten pregnant. If you have regular periods / cycles or if there are any patterns you notice, tell them that too.
It is likely that all they'll do is refer you to your local gynae ward / service.
If they are willing, ask to have your day 3 and 21 bloods taken. This is blood tests on day 3 and 21 of your cycles to measure progesterone and it will tell you if you're ovulating.
Start tracking cycles if you don't already. How long are they? Note when you dtd. You can also measure cervical mucus which may help. There are apps to help with this.
When you get to your referral appointment, it is likely that they will redo your blood tests and send your DP for sperm analysis. The fact that he has a dc won't matter at this stage - what they want to establish is if there are any issues with you or him so that they can plan the best way forward for you.
From there, you will be presented with options. It could be further tests, hormone tablets / injections, ICSI or IVF.
Take your time. I would strongly suggest writing down any questions you want to ask and taking them with you. I took DP with me and had him ask the questions and write down the answers as I got flustered and couldn't concentrate.
Once you have a clearer idea what you're dealing with, I would look if there is an active thread here to join (I have PCOS and found the pcos thread amazing as people posted about what their doctors were doing, what worked for them and were just generally supportive when you needed it)
Good luck