Unless you're talking about index funds, mutual funds with similar objectives will not necessarily own the same securities in the same proportions. Fund managers employ different strategies, and may make different judgements about whether specific companies are good investments.
So splitting your money across 2-3 funds with similar objectives provides some diversification, and therefore some risk reduction -- if one fund makes a bad investment, it's unlikely to be duplicated in the other.
But like all diversification, it also tends to smooth out all results. Protecting against the consequences of bad decisions also means you don't get the full benefit of good decisions.
That said, fund managers are generally basing their decisions on the same raw data about the securities they invest in. Blue-chip stock funds will likely be very similar, since there aren't that many different companies in the category, and their fundamentals are well understood. On the other hand, there are many small cap stocks, and it's hard to research all of them, so small cap funds will tend to be more varied in their portfolios; investing in multiple funds may allow you to get a wider cross section of the market.
There are services like Morningstar that can compare mutual funds, telling you how similar their portfolios are. That way you can avoid investing in funds that are too similar, since there's little diversification benefit.