"Bhadra" means 'beneficial' in Sanskrit.
"Danda" means "stick" in Sanskrit.
"Tri" means "three", "Kona" means "angle", and "Trikona" means "triangle" in Sanskrit The straight legs with the floor between the feet resemble the three sides of a triangle in this asana. Hence the name.
"Veera" in Sanskrit means "brave". The way a brave man takes his position while attacking his enemy, the similar position is formed in this asana. Hence it is called as "Veerasana", the pose of a Warrior.
"Vriksha" means "tree" in Sanskrit. In the final position of this asana, you have to stand still like a tree.