Can I treat all my three residential rental properties as a combined business? Two are in the same state. They are managed by different management companies. Thanks.

Yes, you can generally treat all residential rental properties as a single combined business for tax purposes, but it depends on management and reporting. 

• The IRS allows grouping if properties are operated similarly. Different property managers and differing states don't disqualify aggregation, but you must show a common business purpose and active participation in decision-making.
• You can aggregate your properties for the Qualified Business Income (QBI) deduction under Section 199A, which simplifies reporting and maximizes deductions.
•  To avoid passive activity loss limitations, qualify as a real estate professional and meet material participation requirements. 

For more detailed information, you can visit the IRS page on rental income and the TurboTax guide on rental properties.