Question

why electric towers are in the shape of triangle

Answer 1

to get more height that is the reason

Answer 2

They do it because a triangle has more lateral strength than a square. for instance if you were to build a triangle from 2x4''s and stand it up with the point up and then build a square from the same materials the triangle will be much more supportive if you push down on the top point. The square will tend to try to sway toward one of the vertical sides. Unless you reinforce the corners, by using the strength of, guess what, a Triangle. 
If you look at most bridges and virtually any load bearing structure you find find them supported by triangles. HOPE IT HELPS!!!