3 pay & go are currently sending out text messages to their customers, advising them of claimed improvements to their terms for their 'stretch' offers.
They give a URL which unfortunately only works from your mobile phone.
IF you use your 3 mobile you can view this
here.
This takes you to headings for: "Double your mins & texts"; "Get more mins & texts"; and "Triple your mins & texts".
These offers are graduated, according to the size of your top up & stretch.
Unfortunately, I can neither get the link to work from a PC, nor find its equivalent on 'My 3' via a PC.
However, it should work from your 3 mobile phone.