Hi Anonymoose

I'm actually pretty hopeful for you.
The positives:
1) Your mom is seeing a T

2) I'm making an assumption your mom has agreed to this, as I can't imagine a T setting this up without your mom's support (since either she or her insurance is probably paying for it...?)
3) The T is meeting with you before the 3-way meeting with your mom, so you will have an opportunity to share what you need to, and make a plan for the 3-way meeting.
Wow.
Amazing. I mean, I put this out as an option a few times over my life, and my mom slammed that door shut hard every time. So I see it as a positive that this is moving ahead for you.
I'm going to support
Wools suggestion to let the T know that it is important for you to feel emotionally safe at this first meeting.
As nervous as you are, I'm wondering if it's possible your mom is also nervous, as there's a lot at stake for her, and I'm assuming she's agreed to this meeting...As an outsider looking in, I can't imagine her not having some apprehension (unless I am reading the situation altogether wrong).
In the meantime, don't look back or ruminate (at past hurts), and don't worry about the future (what will happen at the meeting or after the meeting), just live in the present moment, and take good care of yourself between now and then (self-care).
How do I prepare?
1) self-care
2) know what you hope to realistically achieve from this
3) you will have limited time in the session prior to the 3 way meeting, so maybe consider having a few main points prepared (written down) ahead of time about what you want to say to enlighten her on your side of the situation
Keep us posted on how the meeting went

.