Aspects of the relationship:

Serials A, B, and C exist as separate publications. Serial A splits and is absorbed by B and C; serial A ceases to exist. This type of relationship is very rare.

Linking entry fields:

Serial A: 580; two fields ragged 785 15

Serial B: one field tagged 780 X6

Serial C: one field tagged 780 X6

See also:

14.2. Chronological relationships (fields 780/785)