Calculating average NDVI [duplicate]

This question already has an answer here:

• How to get “p value” for time series data to detect significant change 1 answer

I would like to calculate average NDVI values. I have 3 values of NDVI; (1) high:243 low:0 (2) high:243 low:0 (3) high:238 low:0

How can I use a a raster calculator to calculate the average valvue