The moving in thing? Bad idea... I would not suggest it at all. I don't see why you need to start a business and move in together. Why is that? I think that would be playing with fire and there is no room for that in poly relationships in the sense that you are talking about....
I suggest you take a step back and look at this. You are in one relationship already, and have realized you love your friend. Okay, that is fine, but be realistic; you are not in a situation to indulge yourself. You are in a position to do a shit load of work. To even have the indulgence to even consider that this might work will take huge amounts of patience and thoughtful process with your partner and this other person. Respect that and do the work.
Threesomes are all fine and dandy, but that is a long shot that will likely not come out of this as it is... so shelf it for the time being.
This is what I would do if I were you.
1. Tell your partner that you have been in love with your friend for years and that it has been manageable until this point but now it isn't and you need his support to see you through to the next stage, whatever that is... ask him to do this with you. This will show him that you are being respectful, have not cheated, have integrity and consideration for his feelings and position on this.
2. Make a list of what you see would work for you in terms of boundaries. Ask him to do the same. Realize that he is starting at ground zero and you have been in this for years. It could take months for him to catch up. Do research, read, study, find a group near you, go to a poly friendly counsellor, do whatever works to help him feel comfortable and safe in this new information.
3. Ask the friend to back right up and concentrate on your partner. They will have to get to know each other in a different light now and that will take time. The friend will have to realize that you will not be available to him until you have established something solid between you and your partner.
4. You won't be able to indulge in the feelings you have right now, so stuff them and wait... practice that and don't let them out until you have been given the go from your partner. Then wait some more until your partner says, "for pities sake, I'm fine, go be with him." If you get to this point you will feel very proud of the work you have done and everything will be right in the world. It's worth it and possible.
5. Look for other ways to have a business and don't move in with this guy...
This is all my opinion of course and you are welcome to take it or leave it. I have done a good bit of work on this kind of stuff and have been here a long time. What you dream of is possible, I have seen it happen. The above is one way to create that that I have seen and experienced working.
Do a search for tags on "NRE," "mono/poly," "foundations" and "lessons"... maybe they will help.