I have 3 pipes under our floor. What is the third for??

could be allsorts mains ,hot and cold c/heating,gas,ect

It could be Main Gas Supply Pipe or Mains Water Pipe dependent on location off Main Stop Cock or Boiler. However if not on ground floor,most likely to be water if you have cold tank in loft. Whatever it is it will likely be 22mm dia.

it could be gas or mains water in or a loop to the domestic hot water

its best to check first it could be hot cold and gas , or flow and return and gas turn on a tap and feel the pipe for vibration of water flowing if flow and return the pipes would be hot when c/heating is turned on

HI, this hard to say with out seeing it, a good clue is size of pipes 22mm or 15mm, this best way is to follow it back see what it goes to, if its in bathroom could be cold feed to toilet or bidit, also could be gaspipe need mor info, any plumber will tell you in 5min best wishes col.