The Agricultural Stress tool uses 3 different narrow band vegetation indices which require bands in very specific locations in the electromagnetic spectrum, so it is not possible to calculate this index with a 3-band RGB image.

True, but also I have the infrared band, actually I have 4 band (R-G-B-I). With the 4-band is not posibel calculate Agricultural Stress? :(

Have a look at the Help on this Agricultural Stress tool. It uses the following indices: The Agricultural Stress Tool uses the following VI categories: Broadband and Narrowband Greenness, to indicate areas of idle cropland, weak vegetation, and healthy crops. Light use efficiency, to show vegetation growth rate. Canopy nitrogen, to estimate relative nitrogen levels. Leaf pigments, to highlight vegetation stress. Canopy water content, to show levels of water stress. Each of these indices use very specific and narrow bands. You would need all the necessary bands to calculate Agricultural Stress. Typically, it requires a hyperspectral image such as AVIRIS to calculate such indices.