Taking 3 greped/awked lines of text

Say I used grep and awk for taking text out of a text file...
I now /have 3 lines of text that I want to combine to make 1 line.

What command could I use to do this?
...
display line 1 twice then
sed s/$line1/$line2/ ????... display line 2 twice then
sed s/$line2/$line3/

sed '
  $!N
  $!N
  s/\n/ /g
 '

makes sense to me thanks