Just received an email informing me of this promotion. When you go to the page, linked, it actually says there are 3 issues for £1.
However, when you call them, use the keyword 'EMAIL' (tell them you received the email promo) and you get an extra issue in addition!
Note that you have to pay (the £1) by direct Debit and you need to cancel the ongoing subscription before the nd of the trial if you don't want to pay the £8.89 per quarter after those first 4 issues.
T3 isn't the only one as they have quite a few '3 for £1' offers, but T3 was the one I went for and so I know you should get the additional one.
This is my first submission to this site, so please excuse any errors I have made in this posting.