I am not exactly sure of what you are asking for. There are 2 guesses as to what you are looking for;
1. You want the mean of 3 columns separately for each row.
TheMean = Mean( :A, :B, :C)
2. You want to find the average of all of the data for 3 columns:
TheMean = ( Col Sum( :A ) + Col Sum( :B ) + Col Sum( :C ) ) /
( Col Number( :A ) + Col Number( :B ) + Col Number( :C ) );
The above piece of code could be shortened to
TheMean = Mean( Col Mean( :A ), Col Mean( :B ), Col Mean( :C ) );
if the number of data points are equal in each of the columns
Jim