This would be a 3×3 matrix when 3 variables are being considered. The sample covariance is useful in judging the reliability of the sample means as estimators and is also useful as an estimate of the population covariance matrix.
A 3×3 matrix emerges when considering three variables. The sample covariance assists in assessing the trustworthiness of sample mean estimators and can be utilized to estimate the population covariance matrix.