PULASKI, Wis. (AP) _ More than 2,000 mourners who filled a school gym to remember five young girls killed by a speeding car heard condolences from President Reagan and a tape of a victim singing ''Could've Been.''

Many at Tuesday night's community service at Pulaski High School hugged and sobbed as the tape recording of rock star Tiffany's song, sung by 12-year-old Nicole Watia, was played.

''The flowers you gave me are just about to die. When I think of what could have been, it makes me want to cry,'' sang the girl's high-pitched voice.

This morning, 1,000 mourners filled filled a church for a funeral of three of the five girls who were killed Friday night.

The Rev. Jordan Kuczwara told the mourners, many of them young, that they could cry today and go back to their lives tomorrow or they could thank God for the gift of the girls' short lives and learn from them.

''Thirteen years young, but old enough to achieve something,'' said the pastor of Assumption of Blessed Virgin Mary Roman Catholic Church.

He said the girls needed no drugs or alcohol, but found happiness in their friends, family and faith. ''Remember, students, don't cry for them. ... Cry for those who are looking for other ways to find happiness,'' Kuczwara said.

Sobs rose as the caskets of Nicole, one month shy of her 13th birthday, and Laura Karcz and Brenda Hylok, both 13, were closed in the church. Separate services were scheduled later today for Jodi Prokop and Jessica McKeefry, both 13.

At Tuesday's gathering, Nicole's version of the song was among the personal mementos the victims' families shared as this village of about 1,900 gathered for the first time since the accident.

''I feel they (the mourners) want something to happen but they don't know what they want to happen,'' said the Rev. Jordan Kuczwara, pastor of Assumption of Blessed Virgin Mary Church.

Police have been puzzled as to why the driver of the 1975 Cadillac, a 17- year-old high school junior, was going an estimated 70 mph down the residential street before it jumped a curb, killing the Glenbrook Elementary seventh-graders in front of Jessica's house.

Investigators planned to check the vehicle for possible mechanical problems and to investigate whether the driver, a diabetic, suffered a medical problem such as a seizure, said Police Chief Reed Woodward.

No alcohol was involved, he said.

Reagan, in a message sent with Rep. Toby Roth, R-Wis., to the village about 15 miles northwest of Green Bay, said, ''Nancy and I join with each and every one of you as you mourn the loss of five precious young lives.

''There are no words to express what you are feeling or the grief that your fellow Americans share with you as a result of this great tragedy.''

Crying girls hugged one another at the high school, where dozens of floral arrangements surrounded the five closed caskets lined at one end of the gym.

James Brawner, Glenbrook Elementary's principal, estimated that more than 2,000 people filed past the caskets to hug the girls' families and offer sympathy.

Collages of photographs depicting each victim's life were placed on easels near each casket.

Among the letters, cards and messages read during the service was a poem on friendship that Laura Karcz's sister, Lisa, said was one of the last her sister had written.

''I need someone to stick to me to the end and I know that you will because you are my friend,'' the poem said.

The Rev. Floyd Grothman, minister at St. John's Lutheran Church in Pulaski, where Jessica's funeral was planned for today, applauded the community for the way it came together in the wake of the tragedy.

''I love Pulaski more. It's a community,'' he said. ''I don't think I saw such a concentration of hugging that I saw Friday night. Pulaski cares. Pulaski hugs.''

Residents also expressed concern for the driver of the car, Scott Karcz, who was listed in critical condition Monday at a Green Bay hospital. Hospital officials said Tuesday his parents had asked that no further reports be given on his condition. Karcz is a distant relative of Laura Karcz.

His 11-year-old sister, Tina, who was a passenger in the car, was also seriously injured.

''I'm sensing among the kids that they're feeling bad for the Karcz family,'' said Guy David, principal of the high school that Scott Karcz attends.