The answer provided by Richard is correct for RHEL 7 as well.
I would have just added this as a comment to his answer, but I don't have sufficient reputation yet.
According to the documentation, the correct place for user modified systemd unit files to reside is /etc/systemd/system.
I can confirm that by placing the dhcpd.service file in /etc/systemd/system and modifying the ExecStart line in this file to include the desired interface(s) as he has shown in his answer works.
To expand a little on his answer:
After making the changes and starting the service, you can query it's status with:
systemctl status dhcpd.service
I get the following output confirming that it's listening on the correct interface, and only the one that I want.
systemctl status dhcpd.service
● dhcpd.service - DHCPv4 Server Daemon Loaded: loaded
(/etc/systemd/system/dhcpd.service; disabled; vendor preset: disabled)
Active: active (running) since Mon 2011-05-02 03:32:11 CDT; 5s ago
Docs: man:dhcpd(8)
man:dhcpd.conf(5) Main PID: 27814 (dhcpd) Status: "Dispatching packets..." CGroup: /system.slice/dhcpd.service
└─27814 /usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid enp3s0
May 02 03:32:11 host.example dhcpd[27814]: Copyright 2004-2013
Internet Systems Consortium. May 02 03:32:11 host.example
dhcpd[27814]: All rights reserved. May 02 03:32:11 host.example
dhcpd[27814]: For info, please visit
https://www.isc.org/software/dhcp/ May 02 03:32:11 host.example
dhcpd[27814]: Wrote 0 deleted host decls to leases file. May 02
03:32:11 host.example dhcpd[27814]: Wrote 0 new dynamic host decls to
leases file. May 02 03:32:11 host.example dhcpd[27814]: Wrote 0 leases
to leases file. May 02 03:32:11 host.example dhcpd[27814]: Listening
on LPF/enp3s0/00:1e:0b:50:f7:64/10.0.0.0/24 May 02 03:32:11
host.example dhcpd[27814]: Sending on
LPF/enp3s0/00:1e:0b:50:f7:64/10.0.0.0/24 May 02 03:32:11 host.example
dhcpd[27814]: Sending on Socket/fallback/fallback-net May 02
03:32:11 host.example systemd[1]: Started DHCPv4 Server Daemon.
/etc/systemd/system/dhcpd.service
[Unit] Description=DHCPv4 Server Daemon
Documentation=man:dhcpd(8) man:dhcpd.conf(5)
Wants=network-online.target After=network-online.target
After=time-sync.target
[Service]
Type=notify
ExecStart=/usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid enp3s0
[Install]
WantedBy=multi-user.target
Tested on RHEL 7.6 x86_64 with dhcp-4.2.5-68.el7