This is a special syntax, expanded by bash. It also works for zsh.
According to the bash man page (section HISTORY EXPANSION), the pattern
expands as following:
- The event designator
!# refers to the entire command line typed so far which is
curl http://beyondgrep.com/ack-2.14-single-file > ~/bin/ack && chmod 0755
: splits between the event designator (this case the entire line)
and the word designator (selects a sub-part)
- the word designator
3 which selects the third word/argument (counting of words starts at zero), in this case
The final command line (usually displayed before executed) is:
curl http://beyondgrep.com/ack-2.14-single-file > ~/bin/ack && chmod 0755 ~/bin/ack.
For details, see the bash manual or very similar the zsh manual