The Celtics do not have the pieces to land Howard. Period. However, the Celtics easily could be the third team in the trade.
Something like this (not sure Orlando does this, but I think the Celtics and Hawks do it)
Boston - Josh Smith, JJ Redick
Atlanta - Dwight Howard, Ray Allen, Hedo Turkoglu
Orlando - Joe Johnson, Kevin Garnett, Jeff Teague, ATL 1st
This would also work (not sure all teams would do it, but reasonably fair for all 3)
Boston - Kris Humphries, Anthony Morrow
New Jersey - Dwight Howard, Hedo Turkoglu
Orlando - Brook Lopez, Ray Allen, Jordan Farmar, NJ 1st, NJ 1st