No, Wallace's contract is not the issue for "signing" a third star. Star players command max or near max deals in free agency. To sign three outright in free agency, you'd have to have no more than about $8 million in salary and cap holds, which includes draft picks and unused roster spots. If , in the summer of 2015 for example, we had no draft picks, and no one under contract aside from say Olynyk or Sullinger and our #17 draft pick from this year, we could sign Rondo, Love, a third star, and fill the rest of the roster with minimum contracts, and offer one player a "room" exception contract. To get three stars under the current CBA requires too much cap room to have anyone else on your roster.
So yes in 2015, it's Wallace, but it's also our #6 overall pick, Green if he doesn't opt out, Bradley if he comes back, even at a cheap price of $4 million per year, one of Sullinger or Olynyk, next year's picks, anyone else we get this year who we want to keep in 2015, etc. In 2014, it's also Bass, Faverani, Anthony, etc. To get three stars, sign-and-trades are the way to go, because those let you exceed the soft cap, so you can have salary allocated to role players. And in 2015, Wallace will be an expiring contract to include in such a sign-and-trade, because we'll need to send outgoing salary to acquire the free agent, and other teams will want that salary to be for no more than one year.