Max data for a Chart on Rpi Zero W2

General
3

3 days at 1/minute is 4320 samples. With three lines that makes 12960 data points. If the file takes up 3MB (I assume that you mean Mega Bytes, not what you wrote, which is milli bits) then that means each data point is 700 bytes long in the file.

Have you looked in the file to see what is actually there to see why it is so long?

Edit, sorry, got that wrong, it is 230 bytes per sample, which still does not make sense.