You should understand on which interfaces you can configure DHCP services for each deployment mode.
Bridge Mode: The MGMT interface is the only interface that can be configured as a DHCP server.
Software Switch: By default all interfaces except the MGMT interface are L2 interfaces that are part of the software switch. The WAN, LAN1, and LAN2 interfaces must be included in the switch configuration. You can, if desired, remove LAN3 from the switch configuration. After you remove the LAN3 interface from the software switch configuration, you can change it to an L3 interface, assign it an IPv4 address, and then enable DHCP services on the interface.