Q: how to merge two apple IDs...one mac.com and one me.com

Unfortunately, you cannot merge Apple IDs. See this Apple article:

Frequently asked questions about Apple ID - Apple Support

Are they actually the same ID?

I for example started off with mac.com which became me.com and is now iCloud.com, as such I have email addresses in all three domains and these are actually all the same account with the same content.

If they are genuinely different accounts then what content are you referring to? If emails, then you need to setup Apple Mail with both accounts and drag emails from one to the other until they are all in a single Inbox. The same applies to contacts. For calendars, you might be able to do the following, again setup both accounts so both calendars show up at the same time - they will be coloured differently. Go through all the entries for the account you want to move away from and edit each entry to 'move' it to the other calendar.

For calendars you need to export to your Mac then to the other iCloud account by following first the second then the first set of numbered instructions here:

http://www.wilmut.webspace.virginmedia.com/notes/icloudmovecalendars.html

For contacts you would need to adapt Winston Churchill's method here:

Re: How to transfer Address Book to ICloud?

to transfer the contacts to your Mac, then follow it again to transfer them to the iCloud account.