How to change 2x11 to 3x11 on mtb
Sorry to say that you can't if you want to stay with SRAM.

Looking at the list of SRAM cranksets:


Mountain: https://www.sram.com/sram/mountain/component/cranksets
Road: https://www.sram.com/sram/road/component/cranksets


There are none that have 11sp and 3 chainrings.

But looking at the spec of the bike that I think you have:
http://www.halfords.com/cycling/bikes/mountain-bikes/carrera-fury-mountain-bike-2015

It appears that it's 2x10, rather than 2x11 (it says 20 gears "SRAM X5 20 speed").  Can you check if it's really 11sp that you need?

If you need 3x10, then you'll need to change the crankset and the left shifter - and possibly the front derailleur - though you can probably get away without it.

You can find a matching X5 SRAM 3x10 crankset online pretty easily e.g. http://www.ebay.com/itm/SRAM-X5-GXP-175-10-Speed-44-33-22-Crankset-/272056712049

Note that going to 3x10 does not give you easier gears.  It just gives you smaller jumps between gears.  If you want easier gears, then there is a much simpler and cheaper solution: get a smaller small chainring, and/or a cassette with a larger big cog.