You as the employer are required to match each employee's salary reduction contribution up to 3% of the employee's wages. You can make employer contributions instead and then you don't need to abide by the 3% rule. Here is what is says in an IRS article: SIMPLE IRA contributions
You enter the SIMPLE IRA contribution in TurboTax in the business section on the screen that says Let's gather your business info, choose the Self-employed Retirement entry:
Later, you will come to a screen where you can enter your SIMPLE IRA contribution: