As we know, both Jupiter and Neptune have great spot like features in South hemisphere, and Saturn has dragon storm in south hemisphere, is it true that great spot like features appear in South hemisphere more frequently? If so, what is the reason?
Given that we have very few examples, we could not, statistically, draw any significance from any apparent bias. It's also hard to see any theoretical basis for expecting a bias, so my gut feeling would be there probably is no bias.
It's also worth remembering that human observations of these things are essentially a very brief snapshot in a huge period during which we have no idea about what may have been happening. So again the statistical significance of what happens at the moment needs to be carefully considered.
Hubble observations in 1994 revealed that Great Dark Spot on Neptune observed by Voyager 2 had disappeared. In 1998, further observations revealed the formation of a similar dark spot in the planet's northern hemisphere, so at least in the case of Neptune and Saturn (see adrianmcmenamin's answer), both hemispheres are capable of forming such systems.