To get Sims to marry in "The Sims 3," you need to persuade the potential couple to gradually develop their relationship. Depending on the Sims, this could take several hours of gameplay time.
You have an easier time making two Sims fall in love and get married if they have the right traits. Sims who possess the Charismatic, Friendly, Flirty and Hopeless Romantic traits fall in love the most easily. Choose two Sims who have these traits to make the job easier.
Start by developing a friendship between your Sims. Once two Sims are friends, you can use functions, such as Flirt or Confess Attraction, to develop a romantic interest between them. Be careful not to use these advances too early, or the relationship could be damaged.
Once you have developed a romantic interest between two Sims, instruct one of them to ask the other Sim to go steady. This changes the status of the relationship into a boyfriend/girlfriend relationship.
When your Sims are going steady, you can propose marriage. For the best possible chance of success, wait until the relationship score is as high as possible, and make sure that both Sims are in a good mood.
The final step is to hold the wedding. You can plan the wedding party by using the Throw Party option. Other Sims in the neighborhood can attend the wedding to celebrate with the happy couple.