When Roberta Gainor first discovered she was pregnant with twins she was "thrilled." Four years, a deluge of diapers later and pregnant with a second set of twins, she cried, "Why me?"

Although twins occur just about once in every 90 births, Gainor again beat the odds three years ago and bore her third set of twins.

"It scared me," admits Gainor, 38, who lives on Bolling Air Force Base. "I didn't know if I could handle it."

A plump, comfortable woman whose maternal and paternal grandmothers each had twins, Gainor became a twin-care expert through experience.

She learned, for example, to buy baby formular by the case and assign each toddler exclusive rights: to one of her knees to help reduce rivalry for affection.

By the time she bore her third set, Gainor had devised an ideal feeding routine. She sat in a rocking chair and fed one twin in her arms, while her foot rocked the other in an infant seat.

Gainor advised other mothers of twins to first, join their local Mothers of Twins club, a "sanity-saving" organization with 8,000 members nationwide. Each club sponsors a clothing and equipment exchange and holds discussions on rearing multiple-birth children.

"It made me realize I wasn't the only woman in this world who had two little babies," Gainor says. "We exchanged some good ideas."

Two Mothers of Twins members, both Boston nurses, have compiled twin-raising tips gleaned from interviews with more than 100 other mothers of twins. Their book, "The Care of Twin Children," is published by the Center for the Study of Multiple Gestation, run by Dr. Louis Keith of Chicago and his identical twin brother, Donal Keith of Reston, Va.

Unlike most books on twins which deal with psychological aspects of twin-rearing, their book concentrates on practical concerns, according to authors Rosemary Theroux and Josephine Tingley.

"When I was pregnant, I went to the library and there was nothing on how to care for twins," Theroux recalls. "I was interested in breast-feeding my twins, but my obstetrician told me it was impossible.

"Later, after I joined the Mothers of Twins club, I found out that it is possible."

The book covers more than 45 others topics, including traveling, diapering, bathing, toys, toilet-training and illness, plus a special chapter on parenting triplets. CAPTION: Illustrations 1 through 4, no caption, Drawings by Mary Shepard, from "Mary Poppins Comes Back" and "Mary Poppins in the Park," Harcourt Brace Jovanovich, and by Bobbi Bongard, from "A Sign of Relief," Bantam Books.