## [Perl 6](http://perl6.org/), <strike>34</strike> 32 bytes

 {[~] flat roundrobin |$_».comb}

<strike>

 {roundrobin(|$_».comb).flat.join}
</strike>

A lambda that takes an array of strings as argument, and returns a string.

([Try it online](https://ideone.com/3yFIAL))