The third movie in the Dragonball Z movie franchise isn't all that special. It's not horribly bad, but it just feels like a pale imitation of the actual series.
This movie is the first movie to involve Saiyans as an enemy. It's also the only DBZ movie to feature the Oozaru form of a Saiyan (pictured below).
The plot of the movie is simple, I guess. A bunch of Saiyans come to earth to plant a tree called the Tree of Might. The Tree of Might is this legendary tree that sucks all life out of the planet it inhabits to grow this special fruit that is supposed to make the consumer all-powerful.
One thing that really annoys me about this movie is the fact that the primary villain Turles looks like Goku. There's no familial relation between the two, but they just happen to look the same. Couldn't the artists have been a bit more creative with the villain's appearance?
At any rate, I think this is probably the weakest of the first three movies because there just isn't a lot of originality. At least the first two movies have a unique enemy. The entire first saga of Dragonball Z is devoted to the Saiyans so why have a movie about it?
Of course with most DBZ movies this one can't fit into the series at all.
I'd also like to note that this is one of the last movies to really have a decent running time. This one, like World's Strongest, is an hour in length. One of the biggest problems with the DBZ movies are the short running times, but at least that isn't one of this movie's problems. This would the last one of any real length until Broly: the Legendary Super Saiyan which is a whopping 70 minutes.