Group Divisible Designs with Two Association Classes

We investigate group divisible designs with two association classes (known as GDDS, GADs or PBIBDs) with block size 3 and unequal size groups. We completely determine the necessary and sufficient conditions for groups with size vector \((n, 1)\) for any \(n \geq 3\), and \((n, 2, 1)\) for \(n \in \{2, 3, \ldots, 6\}\). We also have some general results for \((n_1, n_2, n_3)\).