In my experience the most difficult thing to change if you are new to web-design is the main structure of the page. If you want three columns, then I would definitely suggest looking for something that already has those three columns.
It is very easy to change spacing, fonts, colours etc. Also, you can hunt for features on other themes that you like and incorporate them into your own concoction.
I spent a few sleepless nights trying to hack a two-column theme to make a three-column theme, and everything went haywire (the reason, I discovered belatedly, was that I had chosen to base my project on a theme where the author had not been particularly faithful to the WordPress Loop). I scrapped the whole thing and started fresh with the template pages from the 'Classic' theme. I redid all of the CSS, starting with the three column structure and then modifying from there. I got what I wanted, but next time I am going to the WordPress Codex and reading EVERYTHING about Blog Design and Layout before I even start.
Ugh... I'm babbling!
Right. If you can't find an appropriate theme to use as a starting point I could send you the three-column thingy I made... just be warned: it has not been prepared for public release (being my first attempt, it was done in a caffeine daze without sufficient preparation, documentation, or sleep).
my email is xiao3ka1gmail<dot>com
Kat, the Twat