(Reuters) - Three major U.S. health insurers that provide coverage to millions of Americans said they would keep some protections included in President Barack Obama's healthcare overhaul regardless of how the Supreme Court rules on the law.

The Supreme Court is expected to decide later this month whether to strike down all or portions of the law, Obama's signature domestic policy achievement that was passed in 2010.

UnitedHealth Group Inc, the largest health insurer by market value, announced its intention early on Monday to preserve some of the provisions that have already taken effect. Rivals Aetna Inc and Humana Inc made similar pledges later in the day.

All three said they would still offer coverage for dependents up to age 26 under their parents' plans. The companies will also continue to offer certain preventive healthcare services without out-of-pocket cost-sharing.

"The protections we are voluntarily extending are good for people's health, promote broader access to quality care and contribute to helping control rising health care costs," UnitedHealth Chief Executive Officer Stephen Hemsley said in a statement. "These provisions make sense for the people we serve and it is important to ensure they know these provisions will continue."

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