Is it possible to make the k2 content module hide the first item from category source? And then show only number 2 and 3? I see that there is no option for that, but perhaps some template overriding and php-coding could make it work? I just cant see how.

:-)

You can use CSS :first-child www.w3schools.com/cssref/sel_firstchild.asp will do the trick.
Other wise you will need to override the module template.