What percentage of the roof area is it? If a large proportion it'll make more difference than if a small proportion of the total roof area. On average, you will save some energy by using triple glazing (but it will be a long time before it saves you any money).
Also, not all double or triple glazing is the same. The U-value will vary related to several factors including the external frame, thickness of glass, overall thickness of glazed unit, and type of gas the glazed unit is filled with.
If cost is an issue you might be better off increasing the amount of insulation in your roof (or elsewhere). It might be cheaper to do that to achieve the same overall result than spend a fortune on triple glazing. If money is no object, get the best standard of glazing possible AND stick more insulation in.
As of June 15th you are going to have to meet more stringent Part L requirements anyway. Take a look at
https://www.quadrantai.co.uk/latest-news/industry/part-l-transitional-provisions-for-domestic-extensions-2022/#:~:text=The new edition of the,thickness of insulation within extensions.