You can define the wavelengths by editing the header file and adding them. This can be found under File ->Edit ENVI Header ->Edit Attributes ->Wavelengths. This is the easy part. The more difficult part is knowing what bands you have if you only have a 3 band file. You should ask the data provider which 3 bands have been supplied. If you display the image and it looks like a true color image (as it would appear to the eye), then I would assume you were given the visible bands of blue, green and red. If you display the bands and vegetated areas appear red, then you were probably given the green, red, and nir bands to make a 'false color composite'. You might also want to know if the image comes from Landsat TM-5 or ETM-7.

hi, i think we're doing the same thing. i'n quite new to envi too. in fact, this is my first post!  in regard to the ndvi analysis,t he tools available in envi (and i believe any tool connected to ndvi) will require a near infrared band apart from the basic rgb. usually it is the 4th band (in landsat 7) because vegetation has its reflectance a t its highest in the red to NIR bands. the tool can be accessed in transform>NDVI, or from the spear tools menu.make sure that you have the bands red and near infrared ready when deriving ndvi!   I have done up to the step wherein i have the ndvi, and now i am looking for the succeeding steps and tools after deriving ndvi. i have a rough idea on the workflow but i still don't know the specific steps to take. I've been browisng the forum, and i'll create a new thread since none of the posts have the exact topic i wish to know more about.