MEDIA, Pa. (AP) _ Sylvia Seegrist was legally insane when she gunned down 10 people in a suburban shopping mall last fall and killed three of them, her attorney said today in summarizing her insanity defense.

''You must distinguish between the act and the person,'' Ruth Shafer told the Delaware County Court jurors, who began their deliberations at about 12:30 p.m. She asked them to find Miss Seegrist innocent by reason of insanity.

''Certainly, Sylvia Seegrist on Oct. 30 in camouflage clothes and in fanatical fashion went in and killed people,'' Ms. Shafer said. ''Just as surely as she did that, she was not criminally responsible.

''She did not know what she was doing. She did say afterward, 'How many did I kill?' but she didn't know what she was saying. Sylvia Seegrist was legally insane when she killed. She did not know the nature or the quality of her acts.''

Assistant District Attorney William Ryan, however, portrayed Miss Seegrist as a deliberate killer whose motive for the rampage was her anger at society.

''She wanted to kill on Oct. 30,'' he said. ''She wanted to get back at us, at society, and she wanted to be famous. ... And she let the whole world know by committing this outrageous crime. She knew what she was doing and she knew it was wrong.''

Ryan acknowledged that Miss Seegrist has a mental illness, but accused the defense of throwing the word ''delusion'' around ''like a Frisbee.''

''She didn't have delusions. She knew the nature of her acts. She did it intentionally.''

On Wednesday, a leading forensic psychiatrist agreed with the prosecution contention that Miss Seegrist, 25, of Crum Lynne, who is charged with murdering three people and wounding seven at Springfield Mall, knew what she was doing.

''She saw herself as not having been a success and having a cause. The shootings were a way of achieving identity and drawing attention to herself,'' said prosecution witness Dr. Park Elliot Dietz of Charlottesville, Va.

Dietz was the chief prosecution witness at the 1982 trial of John Hinckley Jr., who was acquitted by reason of insanity of shooting President Reagan.

Dietz said he agreed with the defense that Miss Seegrist suffers from a severe thought disorder combined with manic-depression.

But he said he could not agree she was out of touch with reality or delusional when she opened fire. He cited her organization in executing the crime and her remarks to authorities afterward.

''She inserted a loaded magazine ... and walked at a fast pace to the mall, where she managed to shoot as many people as possible, and we know she aimed because she chased Tiffany Wootson around the directory in the lobby'' before wounding the 10-year-old, Dietz said.

''Managing to hit 10 people with 30 rounds indicates some effectiveness in organization,'' he said.

After the shooting, Miss Seegrist said, ''Look, you know I've done something wrong. You should have me shot,'' and ''How many did I kill?'' according to testimony.

The remarks illustrate her grasp on reality, Dietz said. He added some of her statements were made to ''amuse or for shock value.''

But Ms. Shafer contended that Miss Seegrist, who had been hospitalized in mental institutions 15 times in 10 years, was ''very seriously ill.''