example.pp

$value = '2'

if $value == '1' {
  $conditional_require = 'example1'
} elsif $value == '3' {
  $conditional_require = 'example3'
} else {
  $conditional_require = []
}

notify { 'example1': }
notify { 'example2': require => Notify[$conditional_require] }
notify { 'example3': require => Notify[$conditional_require] }