On your question about medication for depression, that's a serious business and it's really not a good idea to self-medicate. There are many alternatives and not all of them work for everyone. Usually a doctor has to just try one combination of drugs after another until one finally works. And once you are on medication your doctor will want to see you regularly to keep track of how you are doing. Sometimes the meds need to be changed.
Here in the UK it's the doctor and not the therapist who prescribes medication for depression. If you are feeling so miserable, it's crazy that anyone should tell you you're fine. No you aren't! I would go see another doctor.
On the other stuff - pot, porn, interest in a threesome - it seems to me that those are all ways you have found to react to a common problem. I think any boy who is being abused by his own mother would be incredibly traumatized and desperate to find some way to cope. In many cases the boy just closes down emotionally and in a way "refuses" to feel anymore.
As he grows up his coping strategies will just come along with him into adulthood. In your case, it looks like what you are doing is trying to find ways to feel again, especially where enjoyment of sex is concerned. Any of these might help temporarily, but they aren't a solution and never will be; each time you will just sink back to where you were before, and that will leave you searching for yet other ways to feel. You refer to depression and apathy and I think that about sums it up.
The good news is that at least you recognize you have a problem. It took me years
to admit that, and then a long time to seek the help I needed. One day I would deny I needed help, and the next I would be afraid I would be blamed, ridiculed, shamed, or told I was crazy. You are so right in seeing a need for healthier activities in your life.
I am no therapist or doctor, but I think item numero uno needs to be dealing with the depression. Survivors often find it difficult to ask for what they need, but if one doctor tells you you're fine, you just have to insist you are NOT. If that doctor won't help you, find another doctor.
Try not to judge yourself for your feelings. I have in mind what you say here:
I think my problem is that I dont know whats normal and what isn't. I dont know what the threshold should be. I need someone to reset my moral compass.
I don't think it's abnormal to be fascinated with the idea of a threesome, or if it is, there are a LOT of abnormal guys out there!!!!! Your problem is the same as what many other guys struggle with: a lack of good boundaries and a feeling of inability to set such boundaries. It's not unique to you, and it's a result of abuse that any therapist would expect to see.
We all struggle with the problem of what's "normal", but my response to that is "fuck normal". We are all different and have different interests and needs sexually. In my opinion what we need to do is ask ourselves 1) am I being totally honest with myself about my sexual feelings and issues?, 2) am I being open and responsible with my sexual partner(s) and attentive to their needs as well?, and 3) do I feel fulfilled sexually? If we can answer yes to all those questions we should be okay.
Just some thoughts, my friend. You raise a lot of important topics here.