I have 3 tuners on my HTPC and when editing a channel I accidentally split one of the tuners to its own channel. How do I combine it back to the previous setting?

That one's easy.  :)
In Edit Channels (or, in the Guide, too:  right-click channel name > Edit Channel) > select unwanted channel listing > Edit listings.  Re-enter the Guide listing (channel or network callsign or name), using one that exactly matches the listing you want to keep. 
When asked, select the (default, IIRC) selection to combine channels and remove from Guide (it'll roll the unwanted 'channel' into the other one.)  Save your way out.

HTH,