& is not required for systemd

scripts/3proxy.service

@@ -5,7 +5,7 @@ After=network.target
 
 [Service]
 Environment=CONFIGFILE=/etc/3proxy/3proxy.cfg
-ExecStart=/bin/3proxy ${CONFIGFILE} &
+ExecStart=/bin/3proxy ${CONFIGFILE}
 ExecReload=/bin/kill -SIGUSR1 $MAINPID
 KillMode=process
 Restart=on-failure