If you want to use the Trade Machine for BYC players, just do this steps.
This is an example for a two team trade:
1) Choose the teams, your team (Celtics) being the one with the BYC player.
2) Choose the players you want to trade for (the other team).
3) Forget about the specific players you want to use in a trade (your team) and just select salaries that will add up to what half of the BYC's salary is plus any other player that will be involved in the trade.
4) Check trade, if it's successful, it means the trade works for the Celtics.
5) Do the same process above (leave the players you want to trade for intact) and this time for the Celtics' players simply choose salaries that will add up to the actual salary of the BYC player and any other you want to use in a trade.
6) Check trade, if it's successful, it means the trade works for the other team.
So if steps 4 and 6 are successful, the trade works.
As an example, let's say you want to trade Scal and Baby (4m this year) for Yi and Dooling. The first portion, try trading for Scal and Rondo... if it works, it means that a trade with Scal and Baby works for the Celtics.
Try the trade again, adding JR and Pruitt to Rondo and Scal, and if it works, it means it works for NJ.
That should give you a rough idea if a trade would work with a BYC player without doing too much math.
If you want to do a 3-player trade, I think you can do the same as above without problem. Leave the other teams' players intact, and simply match salaries with your team (BYC player's team).