If I understand correctly, then there is no solution to the problem above. If you divide 12 teams into 4 groups of three then you can only have four rounds before you need to start repeating pairs of teams that have already met at a basket.
This schedule is an example of four such rounds, also it gives a possible fifth round where you would schedule 4 teams to meet at each of three baskets. Of course if you want everyone to play everyone else, then this would mean playing all 6 possible matches at each basket in the fifth round.
Alternatively, it would be possible to have an 11 round schedule (4 groups of 3) where every team played each other exactly twice.
Ian.