No one can pinpoint the beginning or end of a bear market or a bull market. A sensible, long-term approach to investing has always proven to be worthwhile. A well-diversified portfolio allocated between equities and fixed income, according to an investor's age and tolerance for risk, can withstand the volatility of market cycles and preserve and build assets. Equity investments can be allocated according to size, such as large-cap, mid-cap and small-cap. They can further be allocated according to investment style such as growth, value and blend. Both U.S. and international stocks add a further layer of diversification to a portfolio. Mid-cap growth mutual funds are an excellent way to add diversification to a portfolio.These three funds provide investors with the opportunity to gain exposure to the mid-cap growth sector and achieve balanced, risk-adjusted returns over the long term. 

T. Rowe Price Diversified Mid-Cap Growth Fund (PRDMX)

The T. Rowe Price Diversified Mid-Cap Growth Fund ("PRDMX") holds a four-star rating from Morningstar and boasts a consistent performance record, dating back to the fund's inception in 2003. Through March 31, 2016, the fund returned -4.21% for the previous 12 months. However, the results have been very good over the three-year (11.16%), five-year (9.56%) and 10-year (7.56%) periods. The 10-year return of 7.56% includes the market collapse of 2007-2009, proving that this is a fund to own in both bull and bear markets. This mutual fund held $532 million in assets, as of March 31, 2016. It charges an expense ratio of 0.87%, as compared to the mid-cap growth category average of 1.29%. The minimum initial investment is $2,500.

Columbia Mid-Cap Growth Z Fund (CLSPX)

The Columbia Mid-Cap Growth Z Fund ("CLSPX") holds a three-star rating from Morningstar. Although the one-year return through March 31, 2016, was negative 3.04%, the returns were 10.03% for the past three years, 7.14% for the past five years and 6.97% for the past 10 years. The consistent performance of this three-star-rated fund is reflected in the fact that it has shown positive returns for 25 out of the past 30 years, and negative returns in only five of the past 30 years. The fund held $1.9 billion in assets, as of March 31, 2016, and charges an expense ratio of 0.94%. The trailing 12-month yield is 1.11%. There are no minimum initial investment restrictions.

Janus Enterprise D Fund (JANEX)

The Janus Enterprise D Fund ("JANEX") is a moderate mid-cap growth equity fund with a 24-year history. The returns were negative 0.74% for the trailing one-year period, 12.43% for the three-year period, 11.06% for the five-year period and 8.93% for the 10-year period. The Janus Enterprise D Fund is a top performer in the mid-cap growth category, showing strong performance in good and bad markets. Investors seeking a mid-cap fund to hold throughout market cycles can look to this fund as a reliable investment. With a five-star rating from Morningstar, the fund held $9 billion in assets, and it charges an expense ratio of 0.84%. The current yield is 0.24%. The minimum initial investment is $2,500.