The 2+1 comfort seats in the center row have floor mounting points that are evenly placed, as reported by m80IA. As is customary for automobiles in the UK, my Dualiner has the single on the near side. However, I can't say for certain whether or not the Traveliner has third-row seats.
To make it comparable to a traveler, you will need to install rear side windows in addition to the underfloor components for a third row. Some insurance providers may not be willing to cover a vehicle that has undergone such modifications because of the increased risk of an accident. To offset the increased risk to rear passengers in a collision, insurers may need proof that child safety seats were installed according to original equipment manufacturer (OEM) specifications. The DVLA must be informed of the additional seating arrangements.
You could be better off trading this in for a Traveller, but I recommend giving it a thorough inspection first.