ＡとＢの違い = difference between A and B
ＡとＢとＣの違い = difference among A, B and C

Therefore せりとレタスとキャベツの違い is "the difference of parsley, lettuce, and cabbage".

You usually say ＡとＢの違い to mean "the difference between A and B" and ＡとＢとＣの違い to mean "the difference among A, B and C", and I think it would be more natural to interpret せりとレタスとキャベツの違い as "the difference of parsley, lettuce, and cabbage" than "parsley, lettuce and the difference of cabbage".