As in previous games of the franchise, in The Sims 3 players control their own Sims' activities and relationships. The gameplay is open-ended and does not have a defined goal. The sims live in neighbourhoods, now being officially referred to as 'worlds', which can be customized, allowing the player to create their houses, community lots, and sims, although many of these come with the core game.
These worlds are now 'seamless', allowing all sims to move around freely without any loading screen in between lots, as happened in the previous games. Thus, the neighbourhood includes community lots which can be leisure lots (such as parks, gyms, and movie theatres) and job lots (town hall, hospital, businesses). Since the neighbourhood is open, the game includes the "Story Progression" mechanic, which allows all Sims in the neighborhood to autonomously continue their lives without the player ever controlling them. This helps to advance the story of the whole neighbourhood instead of only the active playing units. Sims live for a set duration of time that is adjustable by the player and advances through several life stages (baby, toddler, child, teen, young adult, adult, and elder). Sims can die of old age or they can die prematurely from causes such as fire, starvation, drowning, and electrocution.
The primary world in the game is Sunset Valley (in the console version, the main world is Moonlight Bay), while an additional world called Riverview can be obtained for free. All expansion packs to date (except Generations and Seasons) have included a world, and additional worlds can be bought at The Sims 3 Store for SimPoints. Additionally, Sunset Valley and a few of the other worlds available have some degree of connection to the storyline set up by The Sims and The Sims 2. In-game Sunset Valley is stated to be the same town as the default neighborhood in The Sims, and Pleasantview from The Sims 2, although set twenty five and fifty years earlier, respectively. Several pre-made characters from other Sims games appear throughout the Sims 3's worlds, many of them in younger form.
Career opportunities like working overtime or completing tasks can yield a pay raise, cash bonus, or relationship boost. Challenges occur randomly based on each Sim's lifestyle, like relationships, skills, and jobs. Skill opportunities are the requests by your sim's neighbors or community members for Sims to solve problems using their acquired skills for cash or relationship rewards.
The new Wishes reward system replaces the Wants and Fears system in its predecessor The Sims 2. Fulfilling a Sim's wishes contributes to the Sim's Lifetime Happiness score, allowing players to purchase Lifetime Rewards for the cost of those Lifetime Happiness points.
The game introduces a big change in terms of customization with the "Create-a-Style" tool. In this way, every object or piece of clothing in the game is completely customizable in terms of color (which can be picked from a color wheel), material (plastic, stone, fabric, wood...) or design pattern.