No one can play "The Sims 3" demo anymore as of November 2015 without internal access to the company's files. The original demo released by Electronic Arts was made to be played in-browser on its website; as the site no longer hosts this online demo, it is no longer accessible to the public.
The original demo was a 20-minute teaser for the game and some of its early expansions that presented some of the new features alongside the familiar gameplay elements of the previous games in the series. The demo featured several different ways to play through it, allowing the player to choose from a few archetypal premade characters and go on a few different guided adventures through some of the scenarios made for the main game.
The game itself is still sold on EA's website at a discounted price due to the age of the game. "The Sims 4" is now out, and as such, no further additions are being made to "The Sims 3."
Those who wish to pursue "The Sims 4" can try a demo of the Sim creator on the EA Origin website, where a trial of the full version of the game can also be played for free for 48 hours.