Follow https://github.com/kelseyhightower/kubernetes-the-hard-way/blob/master/docs/07-bootstrapping-etcd.md to bootstrap etcd on the controllers.
Some adoptions have to be considered:
export CONTROLLER_INTERNAL_IPS=("10.3.156.2" "10.3.156.3" "10.3.156.4")
cat <<EOF | sudo tee /etc/systemd/system/etcd.service
[Unit]
Description=etcd
Documentation=https://github.com/coreos
[Service]
Type=notify
ExecStart=/usr/local/bin/etcd \\
--name ${ETCD_NAME} \\
--cert-file=/etc/etcd/kubernetes.pem \\
--key-file=/etc/etcd/kubernetes-key.pem \\
--peer-cert-file=/etc/etcd/kubernetes.pem \\
--peer-key-file=/etc/etcd/kubernetes-key.pem \\
--trusted-ca-file=/etc/etcd/ca.pem \\
--peer-trusted-ca-file=/etc/etcd/ca.pem \\
--peer-client-cert-auth \\
--client-cert-auth \\
--initial-advertise-peer-urls https://${INTERNAL_IP}:2380 \\
--listen-peer-urls https://${INTERNAL_IP}:2380 \\
--listen-client-urls https://${INTERNAL_IP}:2379,https://127.0.0.1:2379 \\
--advertise-client-urls https://${INTERNAL_IP}:2379 \\
--initial-cluster-token etcd-cluster-0 \\
--initial-cluster controller-0=https://${CONTROLLER_INTERNAL_IPS[0]}:2380,controller-1=https://${CONTROLLER_INTERNAL_IPS[1]}:2380,controller-2=https://${CONTROLLER_INTERNAL_IPS[2]}:2380 \\
--initial-cluster-state new \\
--data-dir=/var/lib/etcd
Restart=on-failure
RestartSec=5
[Install]
WantedBy=multi-user.target
EOF