It is 52" with an inch or two extra a few inches away from the corners by pushing the floor down with your mat, not enough for 3x 20" mats.
So the DR can take two 25" mats but that is about it.
If you go to the TT Product Page and click on Compare Features , you get this page :
http://www.tarptent.com/allproducts.html
there you find the full specs of all the TT shelters (apart from the Double Moment)
Under Sleeps you will see several shelters with two numbers , for example Rainbow :1+ or Strato Spire 2 :2-3
The plus or the second number indicates that you can fit in another 20" mat (so 2 inside the Rainbow and 3 inside the SS2)
Look at the StratoSpire 2 , Cloudburst 3 or Rainshadow 2.
(if you don't use trekking poles you can get "substitute" poles for the CB3 and RS2)