计算R中矩阵中列数据的平均值

There is 63x14 matrix data. I want to take the average of three column each from last col to first col. I wrote the script as below. But it shows errors as follows. Also it is not save in a given matrix format. Since I'm a beginner to R, I got help from your site always. Please help me to solve this problem. There is 63x14 matrix data. I want to take the