Which design pattern is best suited for approval process?

Which design pattern is best suited for approval process workflow? For example: A user raises a request for access to a particular application. The request has to go through 3 level of approvals L1, L2, L3. These approvals can be by a person/user or a group of users(i.e., group is an entity like SecurityGroup which has 4-5 members assigned). The design should be flexible enough to include another level of approval if required. Also, there can be scenarios where only L1 and L3 approval is required or only L2 and L3 approval is required. If you could provide a pseudo code that will be good.