OP, since you're just beginning you won't be missing out on much by starting with one fund, such as V 500. I started out similarly, with just one fund in largecap and one fund in bonds. I read some books to learn more about the reasoning behind diversification and asset allocation.
Eventually, I moved toward a "roll my own" format using V500, mid-cap, and small-cap index funds. I suppose I could have just plunked it all into a total US but that didn't exist at the time, and now I don't mind just having 3 funds and keeping it roughly proportional to market capitalization.
Bogleheads wiki has a page about approximating the US total market. What they did there is choose various percentages, and then use Morningstar's 9 boxes of the result to compare it to US total market.
http://www.bogleheads.org/wiki/Approximating_total_stock_marketAt the time that page was written, something like 81% V500, 6% midcap, 13% smallcap was a reasonable simulation. Also, 83% V500 and 17% smallcap was another. You could probably go to Morningstar and use the Instant X-Ray feature to build a fake portfolio with those percentages and compare it to US total market in terms of that 9-box model.
Note that a 33/33/33 proportion would actually weight things far towards midcap and smallcap than the US total market is. That's because the size of largecap companies dwarfs the sizes of midcap and smallcap as a part of the economy.
As I got older, I added things to further diversify ... first a little Euro, then Pacific, then Emerging. But honestly nowadays the task of AA is made much easier by the "total" funds that are now around, and the Target funds that change AA for people. Those are for people who are "hands-off." I think the number of funds you use and how often you rebalance is a decision you'll come to.
I don't believe lots of tinkering is necessary, because one's "risk tolerance" is probably not a fixed number but a range, so an AA has quite a bit of leeway before it gets so far awry that you need to rebalance.