You have to have an active membership to use services including add-ons - see here:
I don't understand the rest of your issue/complaint but if you have a problem you should probably contact 48 support:
Hope this helps.
By the way ... buying three internet add-ons (1GB @ €3 each) seems odd.
Doing this means that they would all have the same one month validity.
Normally when you need an add-on it's better to just buy one, use it and if/when it runs out only then buy another.
Especially when you are buying an add-on to tide you over near the end of a membership month because you have used up some of the membership bundled allowances...
Having read your post again I can't really see where you have grounds for complaint.
Seems to me that the issue is really that you haven't managed your membership and add-on purchases and usage as efficiently as possible. That's not a criticism - just an observation/opinion.