This seems to work.

knoppix@Microknoppix:~$ perl -E '
> @l = qw{ a bb c d e f g h };
> say sub { join q{ }, @_ }->(
>     grep defined, map shift @l, 1 .. 3
>     ) while @l;'
a bb c
d e f
g h
knoppix@Microknoppix:~$
[download]

I also piped the output through hexdump to check that there were no trailing spaces (which you do get without the grep).

Update: Using List::Util::min() to remove the need for the grep.

knoppix@Microknoppix:~$ perl -MList::Util=min -E '
> @l = qw{ a bb c d e f g h };
> say sub { join q{ }, @_ }->(
>     map shift @l, 1 .. min( 3, scalar @l )
>     ) while @l;'
a bb c
d e f
g h
knoppix@Microknoppix:~$
[download]