Hi,
Maybe this is a stupid question but is it possible to re-train my three year old dog  to keep off the sofa and sleep in a cosy bed on the floor or have I left that too late?
Thanks in anticipation. 

Possible, but it could take a lot of work.........

Re-train?

Does this mean she has been "trained" unsuccessfully to do this already or that you want to train her to stay off the sofa from now on?

Sleeping on the sofa is not just about it being more comfortable than a dog bed, but about proximity to you etc.

Yes it is possible, but will take a bit of work. I did it with mine when I moved house some years ago. Though when I moved to where I am now the living room was that small I had to let them back up on the furniture so I could move around