merge time machine backups from different external HDs

So I have a Single MacBook Pro that over the course last few years I have made several (3)time machine backups.
One has mostly dates in 2012 one has mostly dates in 2013 and the latest one has mostly dates in 2014.
The external with the 2014 data is backed up on it it Is large enough it should be able to hold all the backups but I can't manually drag-and-drop the 2012-13 dates over to the 2014's backup folder.
is there any utility, third-party or otherwise where I can merge these so those two other external HD'd can be used for something else

thanks