While there isn't a single "fall in love instantly" cheat, The Sims 3 allows you to manipulate relationship levels through the console. Use these at your own risk, as they can disrupt natural gameplay and cause instability.
testingcheatsenabled true and press Enter.With testing cheats enabled, Shift-click on a Sim and select Modify Relationships. You can manually set the Friendship and Romance bars for any relationship.
Use these commands for precise control (requires Sim IDs, found via the Debug menu while Shift-clicking a Sim):
relationship.add_relationship <SimID1> <SimID2> <score> – Adds points.relationship.set_relationship <SimID1> <SimID2> <score> – Sets exact level.relationship.increase_relationship <SimID1> <SimID2> <score> – Increases by amount.Note: These affect the overall relationship score, not specific Friendship/Romance sub-scores independently.
aging off – Stops aging, giving unlimited time to build romance.motherlode (50,000 Simoleons) or money <amount> – Funds for gifts and dates.