The deal is official now after it appears the Broncos and their AFC West Rivals, the Chiefs, tried to sign him as well:
Denver is giving former Steelers WR Emmanuel Sanders a 3-year, $15M contract. Team announced agreement.— Adam Schefter (@AdamSchefter) March 16, 2014
There may be a grievance filed by the Chiefs though after they had an agreement with Sanders' agent, according to Mike Florio of Pro Football Talk:
If the Chiefs are, as they seem to be, upset by the alleged failure of agent Steve Weinberg to honor an agreement in principle for receiver Emmanuel Sanders, the Chiefs can do something other than complain about it.
While the agreement isn’t binding until it’s in writing, all parties to the Collective Bargaining Agreement have an obligation to negotiate in good faith. Article 4, Section 8(a) imposes that obligation on any team, player, or player agent engaged in contract negotiations.
The Denver Broncos chose to let wide receiver Eric Decker walk in free agency, but it didn't take long to find his replacement. After Decker signed a five-year, $36.25 million contract to join the New york Jets, the Broncos were left to replace 174 catches and 24 touchdowns over the past two seasons.
According to Adam Schefter, they're planning on signing Emmanuel Sanders:
Sanders caught 67 passes and scored six touchdowns for the Steelers last season and since he turns 27 next week, he should be headed into the prime of his career.
After the New England Patriots tried to sign Sanders last offseason, they were rumored to be interested in signing him to a long-term deal this year. Once they re-sign Julian Edelman though, their interest cooled, and now Sanders is joining the team with the best chance of representing the AFC in the Super Bowl.
Sanders visited the Jaguars, Chiefs, 49ers and Buccaneers earlier this week. There is some concern with Sanders' durability, as he has a screw in each foot from injuries he incurred in the 2011 Super Bowl.