Could someone explain how 3 add-ons work with PAYG contracts?

The way I have read it is, you top up £10 then immediately use that £10 to buy the add-on, receive your 3000 texts, 100 mins and data, which last for a month.

Does this mean you don't have any credit left after this? Also, do the add-on allowances expire after the month, or do you keep them until they are used up?

The free 150MB and free texts last 3 months or until used if you top up £10 then spend it its gone!
If you top up online you get the 150mb and a few free txt 30 or 50? for £5 top up which is what i do, the £5 credit lasts till spent
I have not used up allowance yet but they say you get a text prompting you to top up more i did to say my free stuff finishing

alan.w I think you are referring to an old 3 tariff?

Has it changed i had it 4 months its got 5000 free 3 to 3 calls as well as other free stuff and skype calls too, Calls are 25p a min but i got no friends so dont call people from mobile LOL

Yeah, I think they are old/different ones, alan.w but thanks for trying to help.

These are the ones I'm talking about:

http://www.three.co.uk/Pay_As_You_Go/Voice_Add_ons

I did have a look seems i can get them too but if you buy a £10 add-on it will take your £10 top up, maybe worth sticking a bigger top-up on and converting the £10

Do they last for 3 months too then? I just can't seem to find this sort of information on the 3 website, it's rather annoying.

Add-ons last for 30 days. It actually says so on that page you linked to above, under 'About Add-ons'.