If \(\overset{̄}{a} = \hat{i}+\hat{j}+\hat{k},\overset{̄}{b} = \hat{i},\overset{̄}{c} = c_1\hat{i}+c_2\hat{j}+c_3\hat{k}\) with \(c_1 = 1\), \(c_2 = 2\), then value of \(c_3\) such that \(\overset{̄}{a},\overset{̄}{b},\overset{̄}{c}\) are coplanar is ____