In a recent Developer update Jeff Kaplan confirmed that Season 3 would be ending soon. He was vague on the details and did not specify the dates for the end of Season 3.
Blizzard have now confirmed the date and time for the end of Season 3.
Season 3 will officially end on Tuesday, February 21 at 4:00 p.m. PST (or Wednesday, February 22 at 00:00 UTC). Season 4 is currently scheduled to launch one week later on Tuesday, February 28 at 4:00 p.m. PST (or Wednesday, March 1 at 00:00 UTC).
As usual, players will get rewards in terms of competitive points, Player icons for completing season 3. They will be able to use the competitive points to obtain skins within the game.
Here’s the Skill Rating (SR) to Competitive Point (CP) breakdown for Season 3:
SR 1-1499 (Bronze): 100 CP
SR 1500-1999 (Silver): 200 CP
SR 2000-2499 (Gold): 400 CP
SR 2500-2999 (Platinum): 800 CP
SR 3000-3499 (Diamond): 1200 CP
SR 3500-3999 (Master): 2000 CP
SR 4000-5000 (Grandmaster): 3000 CP
You can read the official announcement on the Battlenet forums by Lyrria, a community manager.