Row to column and column to row using awk

Some awk version

awk 1 RS=" |\n" file1  # gnu awk version
awk '{for (i=1;i<=NF;i++) print $i}' file1 # portable version
a
b
c
1
2
3

awk '{printf "%s" (NR%3==0?RS:FS),$1}' file2
a b c
1 2 3

printf "%s" # print pararameter #1 ($1)
NR%3==0?"RS:FS # add extra formatting. Test if line is number 3. If its not, use FS (a blank space), if it is use RS, a new line.
So this adjust the next parameter after every 3 line.

I'd use xargs for this:

$ xargs -n1 < file1
a
b
c
1
2
3

$ xargs -n3 < file2
a b c
1 2 3