In reply to Stone Muppet:
Re: booster within 4 months of first vaccination course - what's generally meant by this is that, if you've only had your primary course (e.g. dose 1 and dose 2 of Pfizer), and it's been more than 4 months since your second dose, then you need to also have had a booster to count as fully vaccinated.
Once you've had your booster (so dose 1, dose 2, then dose 3/booster), there is no further 4 month count down from the booster. So no need for a second booster at this stage.