Twice a year, JD Wetherpoon hold a beer festival. At that festival, third pints are available, and generally people have them in threes, although you can order different quantities.
CAMRA members receive a number of vouchers each year to use at Wetherspoon’s and these may be redeemed against a pint of real ale. Unless you want three thirds as the system doesn’t recognise it as a pint.
I emailed head office and they replied saying yes, vouchers were valid with three thirds (subject to a couple of conditions). This is the letter.