Although there are three, two are only open to the public. It's worth the effort to go round them after your visit to Buckingham Palace.
Three in one.
Buckingham Palace is surrounded by three parks: the St. James’s Park to the west, Green Park to the north and Buckingham Gardens to the east; the latter being the only park which isn't open to the public. You may combine your visit with the palace and the changing of the Royal Guard. If so, take into account that this occurs at 11:30 on a daily basis between May and July or every two days during the other months of the year. After seeing this traditional ceremony, a good choice would be to go for a long and pleasant walk around the surrounding area, in order to get to know the place. On one side, you can go round Green Park; it's a royal park in which austerity is preferred since you'll not find lakes or statues, just trees, grass and peace. It's in the privacy of this spot, where classic duels between knights used to take place years ago, when they either used a sword or a pistol. If you keep walking towards the Thames, you'll find St. James’s Park; the oldest of the royal parks. It's more adorned than its neighbour as it has fountains and a central lake. When you find the lake, follow your instinct and go around the edge until you arrive to the bridge that crosses over it. Once you're on this bridge, take a couple of moments to look over to the east and you've got one of the best views of Buckingham Palace.