Hi the 3 months thing is to do with free giffgaff to giffgaff calls/txts (calls no longer than 1 hour).
Goodybags last for 30 days.
To reset the the 3 month you only need to buy a goodybag direct with debit.credit card or buy airtime credit. Buying a goodybag from airtime credit doesn't count.
You have to top up every 3 months or buy a goodybag every 3 months to keep having free giffgaff to giffgaff calls and messages. There is nor rule for goodybags but after a duration of 6th months without using the sim card in anyway it will become void and will no longer work.
Hope it helps!