If you just want to work with text data and not any binary then the simplest format which comes to my mind is as below
x,y,z:123
In the above format specs,
x = index of the first
y = index of the second
z = index of the third
To read this you would first
-- initialize the value of the full array to nulls or Zero
-- read the text file line by line
--- split the line on ":" , split the left side with "," comma
--- read the individual indexes and set the value for that.
To Write it will be simple as you will be writing the values through the loop.
-- to keep the file smaller, you can skip the null.