Since you neglected to name two of them in the question,
I'll have to list all three in order to be sure of answering it:
-- Vega . . . in Lyra
-- Altair . . . in Aquila
-- Deneb . . . in Cygnus
It is a reference to three bright stars that can be seen in northern hemisphere. Altair in the constellation Aquila, forms the top of the triangle. Vega in the constellation Lyra is the brightest star and is almost directly overhead. Deneb in the constellation Cygnus is the third star.
Vega
VEGA
This stars of the winter triangle are a landmark of the winter night sky and is formed by our brightest night sky star, Sirius, the giant star, Betelgeuse, and the brightest star in Canis Minor, Procyon.
Vega is in the constellation Lyra the Harp. It's also part of the summer triangle.
Deneb is a star in the constellation Cygnus. It is part of the summer triangle. It has no specific Japanese meaning.
Cygnus the swan is best seen in the summer season - it's bright star Deneb is part of the Summer Triangle.
The third triangle number is 6.
Cygnus is a swan - it's a constellation whose brightest star, Deneb, helps make up the Summer Triangle.
The Arab term "Summer comes on the wind of birds refers to the the arrival of birds in the spring to signal the coming of summer, in the same way the summer triangle is a astronomical imaginary triangle that shows in the northern hemisphere at the beginning of spring into summer.
The three stars that make up The Summer Triangle are: Vega, Altair, and Deneb.Look below at Sources and related links to see the stars and constellations of The Summer Triangle.
The supplementary angle of the triangle.