My question involves employment and labor law for the state of: california

We have employees on 4/10 shifts. The full time employees for 4/10s each week. However, we have part time employees also working the 10 hour shifts but only 3 times a week. Is this legal?
For example, we have one employee who works 4/10s one week, then 3 10s the next week. We also have one employee who does 3/10s each week. We have been told this is illegal. From what I have read and researched, it is not illegal and as long as it is voted on by employees it is legal.

Help!