Without including Brown it would be difficult, but I think something like this might work near the deadline (if the Simmons saga extends that long)
Boston - Simmons, Looney
Golden St. - Horford, Smart
Philly - Wiggins, Nesmith, Langford, BOS 1st's (22, 24), GS 1st (earliest they can trade one)
Something like that I think makes for all 3 teams, but can't be done until Smart can be traded on January 20th.
So Boston post-trade
Starters - Simmons, Richardson, Brown, Tatum, R.Williams
Rotation - Schroder, Pritchard, G. Williams, Parker, Kanter, Looney
Deep Bench - Hernangomez, Fernando
If this is still going on towards the end of the year, I can see something with GSW, PHI, and BOS, but a little different. I don't think Phily wants prospects (Nesmith, Langford, Picks), I think they want players who plug in and help win now. The result is Simmons ot GSW though, not to Boston:
Boston: Wiseman
Philly: Wiggins, Schroder (can be traded on Dec 15)
GSW: Simmons
This is less than if Philly had just taken Brogdon and Warren from Indy so maybe you have to add in some picks but this is the general idea that to me, is more workable and provides Philly more what they want. I also don't know how much GSW want Simmons.
Now to get Simmons to Boston, if we are going to give up Smart, why not send him to Philly? This would be contingent on GSW actually wanting Al Horford but this is how I would see it:
Boston: Simmons
Philly: Wiggins, Smart (+ Boston 1st?)
GSW: Horford
I am not sure when Horford can be traded and I know there are base year compensation issues with Smart (his salary did not increase by more than 20% so I think this is OK). Philly probably does not do this as they could probably do better with other teams but I think they prefer this over Wiggins + prospects/picks. And if it came to it, we could add a pick.