If you are paying compensation back from a weekly rest on a daily rest, does it simply get added onto whatever daily rest you are having??
E.g. If you need to pay back 7 hours and you have a 16 hour daily rest does this use up one of your 3 reductions even if your shift was shorter than 13 hours ■■
It would definitely use one of your reduced rests, although I’m not sure if competition needs to be added to a normal rest period and not a reduced, ROG or Tachograph will confirm at some point.
needs to be added to a normal rest period
You can compensate for a reduced weekly rest by adding it to any rest period of 9 hours or longer, although if you add it to any rest period of less than 11 hours then that will count as a reduced daily rest for that day.
From the drivers hours book of knowledge 
“The compensating rest must be attached to a period of rest of at least 9 hours — in effect either a weekly or a daily rest period.”
if you are adding the time to a 9 hr daily break then yes it would use up one of your reduction, as the additional time is to make up for the compensation of the reduced weekly rest period, so you would only have 2 left for that current week 