Each team plays 2 in-group games and 3×2=6 cross-group games: 8 games, so 8 rounds.
Groups: 4–6 meet twice, so they're in different groups; 1–5 meet once, so same group. Round 8 is all cross-group: 3–6 and 2–5 are in different groups. The only split that works: {1,5,6} and {2,3,4}.
Round 8 (= Round 5): 3–6, 2–5, 1–4.
Rounds 4 and 7 must be cross pairs still needing two games: 6 must play 2, so 1–3 and 5–4: Rounds 4 = 7 = 1–3, 5–4, 6–2.
Round 1: 4–6, 1–2, 3–5. Round 2: 4–6, 1–5, 2–3. Round 3: 3–4, 1–2, 5–6. Round 6: 1–6, 2–4, 3–5.