"In addition to the color styles, 9 separate grayscale styles are defined, one for each Landsat 7 bands. The style names are Blue Green Red IR1 IR2 IR3 ThL ThH Pan
These styles select a single spectral band out of the mosaic. The Blue Green and Red styles correspond to the respective visual wavelength bands. The IR1 IR2 and IR3 styles select one of the three infrared bands. ThL and ThH correspond to the thermal band, with either High or Low gain."

Check here http://mapus.jpl.nasa.gov/

We ask the IR3 so band 3 in IR not band 3 in visible