From Raster—Spectral—Principal Component.

Look at the output text in a text editor. Arrange it similar to this and calculate the variance (2^nd row value / sum of 2^nd row). If you wish you can rerun PCA on your original file and only use the first 2 or 3 components (in this example). You could use only 2 but since there are three display bands I would use the first 3.