#Perl 5, 22 bytes

21 plus one for -p:

$_=(-$_,$_+2)[$_%3]/3

Uses 1-based indexing.

Explanation:

-p sets the variable $_ equal to the input. The code then sets it equal to the $_%3th element, divided by 3, of the 0-based list (-$_,$_+2) (where % is modulo). Note that if $_%3 is two, then there is no such element, and the subsequent division by 3 numifies the undefined to 0. -p then prints $_.

Perl 5, 22 bytes

21 plus one for -p:

$_=(-$_,$_+2)[$_%3]/3

Uses 1-based indexing.

Explanation:

-p sets the variable $_ equal to the input. The code then sets it equal to the $_%3th element, divided by 3, of the 0-based list (-$_,$_+2) (where % is modulo). Note that if $_%3 is two, then there is no such element, and the subsequent division by 3 numifies the undefined to 0. -p then prints $_.

#Perl 5, 22 bytes

21 plus one for -p:

$_=(-$_,$_+3)[$_%3]/3

Uses 1-based indexing.

Explanation:

-p sets the variable $_ equal to the input. The code then sets it equal to the $_%3th element, divided by 3, of the 0-based list (-$_,$_+3) (where % is modulo). Note that if $_%3 is two, then there is no such element, and the subsequent division by 3 numifies the undefined to 0. -p then prints $_.

#Perl 5, 22 bytes

21 plus one for -p:

$_=(-$_,$_+2)[$_%3]/3

Uses 1-based indexing.

Explanation:

-p sets the variable $_ equal to the input. The code then sets it equal to the $_%3th element, divided by 3, of the 0-based list (-$_,$_+2) (where % is modulo). Note that if $_%3 is two, then there is no such element, and the subsequent division by 3 numifies the undefined to 0. -p then prints $_.