${string: -3}

要么

${string:(-3)}

(注意第一种形式的空格：和-3)。

请参考Shell Parameter Expansion in the reference manual：

${parameter:offset}
${parameter:offset:length}
 
Expands to up to length characters of parameter starting at the character
specified by offset. If length is omitted,expands to the substring of parameter
starting at the character specified by offset. length and offset are arithmetic
expressions (see Shell Arithmetic). This is referred to as Substring Expansion.
 
If offset evaluates to a number less than zero,the value is used as an offset
from the end of the value of parameter. If length evaluates to a number less than
zero,and parameter is not ‘@’ and not an indexed or associative array,it is
interpreted as an offset from the end of the value of parameter rather than a
number of characters,and the expansion is the characters between the two
offsets. If parameter is ‘@’,the result is length positional parameters
beginning at offset. If parameter is an indexed array name subscripted by ‘@’ or
‘*’,the result is the length members of the array beginning with
${parameter[offset]}. A negative offset is taken relative to one greater than the
maximum index of the specified array. Substring expansion applied to an
associative array produces undefined results.
 
Note that a negative offset must be separated from the colon by at least one
space to avoid being confused with the ‘:-’ expansion. Substring indexing is
zero-based unless the positional parameters are used,in which case the indexing
starts at 1 by default. If offset is 0,and the positional parameters are used,$@ is prefixed to the list.

由于这个答案有一些常规的观点，让我添加一个可能性来解决John Rix的意见;因为他提到，如果你的字符串的长度小于3，$ {string：-3}扩展为空字符串。如果，在这种情况下，你想要扩展字符串，你可以使用：

${string:${#string}<3?0:-3}

这使用？：三元if运算符，可以在Shell Arithmetic中使用;因为如所记录的，偏移是算术表达式，这是有效的。