At some point, there may be two valid solutions. (actually three: one obvious, one less, third one even less.)
Edit: actually four solutions, one found by Foitn & Saeïdryl, which is very related to the even less obvious one. There's a hint for the even less one in the picture.
Edit2 : @123 found the most absolute answer of all time in comments!
Vertices are geometric immaterial points. What about pixels?
Triangles need absolutely three vertices. Everything depends on what smallest green light emitting material you chose to relatively set as one vertice. Answer is between 3(obvious) / 8(rgb mix) / none (pixels are not triangles) / none (green is a very personal perception) / a finite computable lot of possibilities (each powered pixel emits green light, even black ones) / a finite googleplexsquared djillion possibilities if you chose unpowered baryonic matter emitting green light inbetween pixels too / another close to infinity probability if you print the image and count green emitting particles aswell / and some small to very big infinity if you add imaginary vertices, if you go fractal, if you count electronic positions' probability relative to time, over and over, until universe's heat death. Everything is absolutely relative, so sometimes it's easier to say there are 3 green triangles, even if any other answer is correct.