This article explains how to use the knob "preferred" under [edit interfaces] when an interface enabled for dhcp-relay has 3 different subnets.

• Customer has an IRB set up on QFX5110 with 3 subnets on the same IRB.
• A physical server on that VLAN/IRB appears to be requesting DHCP sourcing from one of the subnets. 
• What determines which network a DHCP request goes to?
• Is there a way for force which network a DHCP request goes to even though there is 3 subnets on the same IRB?

By default, the lowest-numbered address on the subnet is the preferred address.

One option to change this behavior or to make DHCP use another IP address in this interface would be configure the knob "preferred":

• Configure this address to be the preferred address on the interface. If you configure more than one address on the same subnet, the preferred source address is chosen by default as the source address when you initiate frame transfers to destinations on the subnet.

For example:

user@switch# set interfaces irb unit X family inet address <ip-address> preferred

Source: https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/preferred-edit-interfaces.html