I'm pretty sure that the "three in a tree" bit is actually "three on the tree", which refers to a three-speed standard transmission with the shifter on the steering column (common in older vehicles, like for instance the '65 pickup in the song, but almost forgotten today in North America).
I'm pretty sure that the "three in a tree" bit is actually "three on the tree", which refers to a three-speed standard transmission with the shifter on the steering column (common in older vehicles, like for instance the '65 pickup in the song, but almost forgotten today in North America).