$date90DaysAgo = (new DateTime())->modify('90 days ago')->format('Y-m-d H:i:s');
foreach(craftelementsUser::find()->lastPasswordChangeDate('<='.$date90DaysAgo)->all() as $user){
    Craft::$app->getUsers()->sendPasswordResetEmail($user);
    $user->newPassword = // generate some temp password, so the user can't login with his/her anymore
    $user->setScenario(User::SCENARIO_PASSWORD);
    Craft::$app->getElements()->saveElement($user);
}