All the advice is valuable advice so far. I'll just add that it's my general impression that there's way too little communication going on, especially way too little positive/productive communication. People are bottling things in as long as possible until they explode. When you have an ache or a doubt or a misgiving, you need to tell your partners about it right away. Somebody has to be the first person to start communicating, so do it yourself if C and D won't.
When you do communicate, come from a place of considerate compassion, and have a care not to demean, accuse, or threaten the other person. Put yourself in the other person's shoes. D is in a harsh place because no one wants to feel unwanted, and he feels unwanted.
On the other hand, D should be doing a whole lot different with these feelings. He should be speaking up right away when he wants or needs something, not make you be a mind-reader, and it's absolutely inexcusable for him to kick you out of the house. I'd say that particular behavior needs to stop immediately, or you really should consider a break-up. Tell him it is not okay for him to do that anymore. He's got to find healthier ways to communicate.
C is probably burned out from being the go-between for you and D. This new girlfriend he has is probably his (dysfunctional) way of trying to escape from that pressure. I would advise you to try not to use C as a go-between anymore. Start talking to D directly. Do it in small bits at first if you have to. But get some communication flowing.
If you ever get the chance, do this communicating when things are relatively peaceful, rather than waiting until a crisis has touched down. Be a good listener when the other person tries to say something. Don't interrupt or try to think of a good retort. Use the time as an opportunity to try to understand the other person.
Communication ought to be done with kindness, and if no one else will do it first, then you be the one to set the example. Even if someone barks at you, determine within yourself that you won't allow yourself to go on the defensive (or counter-offensive). It's very important to try to understand the other person.
Regardless, the kicking you out of the house must stop immediately. Set a hard boundary in that area. And yes, you have a right to ask for other things as well. Ask sooner, rather than later, as later it will detonate and hurt all of you (and it already has).
All that, and read the various other posts on this thread with care. A lot of good advice has been given, and good things to think about.
I'm sorry you are caught in such a wrenching situation.
Love means never having to say, "Put down that meat cleaver!"