I had an account with AskUbuntu and another one with StackOverflow. I have been trying to merge the two of them. How do I go about it?
PS: I just realize I might have created a third account for meta. The login is really confusing.
Well, merging accounts is only done between two accounts on the same site. All of your accounts are on different sites, so merging is not possible.
Stack Exchange's policy is for users to have a different account on each site, but these accounts can be associated. Association links together your accounts, but they're still two independent accounts. Your accounts are already associated (association automatically happens if you use the same login), so nothing further can be done with your accounts.
Your account on every Stack Exchange site is separate, because reputation, badges, favorites, answers, etc. are separate for every site.
For most sites, meta accounts are the same as the account on the parent site. So, your account on meta.askubuntu.com has the same user ID and reputation as your askubuntu.com account. Meta Stack Overflow is different because it is sort of like the "national capital" for Stack Exchange, so its accounts, reputation, and everything else are separate from the primary site.
Each of your individual site accounts are all linked through your Stack Exchange network profile. Once you complete the initial signup for each Stack Exchange site you want to be a member of, you will be logged into it automatically if you've logged into any other Stack Exchange sites, using global login.
If you go to your profile on any site, you can see your associated accounts:
I think the account is the same if you are using OpenID (log with Google, Facebook, etc).
In my case I log in with Facebook and my account is the same at all sites (same personal information, same avatar and 101 points of reputation directly by loging on a new site).