Your model should output 3 logits, one for class_a, one for class_b, and one for class_c.

When you use data from the 1st dataset,

• penalize class_a outputs for samples with class_b and anything_but_a_b labels
• penalize class_b outputs for samples with class_a and anything_but_a_b labels
• penalize class_c outputs for samples with class_a and class_b labels

When you use data from the 2nd dataset,

• penalize class_a outputs for samples with class_c labels
• penalize class_b outputs for samples with class_c labels
• penalize class_c outputs for samples with not_class_c labels