Chi-Squared Analysis

Hi all,

First post here. Apologies if this post isn't in the right place, I haven't got a feel for this place yet.

I'm pretty confused about how to go about performing a pair-wise chi-squared analysis of a table of percentages I have (see image).

Basically, I just need to compare each column to Class A (Class A vs Class B, Class A vs Class C) using a chi-squared test.

Am I right in assuming that the right process is to compute the mean percentage of the two columns I'm comparing, then doing the chi-squared calculation [=(x-AVG)^2/AVG] for each percentage, or am I way off?

Additionally, to do a chi-squared analysis of the whole table, do I do the same procedure as above, but use the average percentage of the whole table?

Thank you very much in advance for any help. I've always had a hard time understanding stats, and after trying this for a while, I've become frustrated.