While there have been rumors and fan wishes for a reunion or sequel, there's nothing concrete. The last "High School Musical" movie was released in 2008.
Vanessa Hudgens herself has not confirmed or denied any involvement in a possible third movie. However, she has expressed interest in reprising her role as Gabriella Montez in the past.
Ultimately, only time will tell if a "High School Musical 3" will happen and if Vanessa Hudgens will be a part of it.