New Cluster: nodes 2 and 3 don't join but rather start their own clusters

Followed the instructions exactly by using copy/paste, changing nothing but the IP addresses involved.

However. when I start the second node using the instructions above, it comes up not as a member of the cluster but isolated.

first host:
±-------------------------±-------------------------------------+
| Variable_name | Value |
±-------------------------±-------------------------------------+
| wsrep_cluster_conf_id | 1 |
| wsrep_cluster_size | 1 |
| wsrep_cluster_state_uuid | f28ae440-cabb-11e7-976c-17bca5fe1b6b |
| wsrep_cluster_status | Primary |
±-------------------------±-------------------------------------+

±-------------------------±-------------------------------------+
| Variable_name | Value |
±-------------------------±-------------------------------------+
| wsrep_cluster_conf_id | 1 |
| wsrep_cluster_size | 1 |
| wsrep_cluster_state_uuid | 9df5f23e-cabc-11e7-b6e2-030d02b0e16b |
| wsrep_cluster_status | Primary |
±-------------------------±-------------------------------------+

Environment: Ubuntu 16.04, fresh install with only apt-get update && apt-get upgrade -y done prior to installing cluster. ufw firewall is disabled

config from host 1:

Template my.cnf for PXC

Edit to your requirements.

[mysqld]
server-id=1
datadir=/var/lib/mysql
socket=/var/run/mysqld/mysqld.sock
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid
log-bin log_slave_updates
expire_logs_days=7

Disabling symbolic-links is recommended to prevent assorted security risks

symbolic-links=0

wsrep_provider=/usr/lib/libgalera_smm.so
wsrep_cluster_name=pxc-cluster
wsrep_cluster_address=gcomm://192.168.175.141,192.168.168.30
wsrep_node_name=pxc1
wsrep_node_address=192.168.175.141
wsrep_sst_method=xtrabackup-v2
wsrep_sst_auth=sstuser:passw0rd
pxc_strict_mode=ENFORCING
binlog_format=ROW
default_storage_engine=InnoDB
innodb_autoinc_lock_mode=2

Template my.cnf for PXC

Edit to your requirements.

[mysqld]
server-id=2
datadir=/var/lib/mysql
socket=/var/run/mysqld/mysqld.sock
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid
log-bin log_slave_updates
expire_logs_days=7

Disabling symbolic-links is recommended to prevent assorted security risks

symbolic-links=0

wsrep_provider=/usr/lib/libgalera_smm.so
wsrep_cluster_name=pxc-cluster
wsrep_cluster_address=gcomm://192.168.175.141,192.168.168.30
wsrep_node_name=pxc2
wsrep_node_address=192.168.168.30
wsrep_sst_method=xtrabackup-v2
wsrep_sst_auth=sstuser:passw0rd
pxc_strict_mode=ENFORCING
binlog_format=ROW
default_storage_engine=InnoDB
innodb_autoinc_lock_mode=2

±--------------±----------+
| user | host |
±--------------±----------+
| mysql.session | localhost |
| mysql.sys | localhost |
| root | localhost |
| sstuser | localhost |
±--------------±----------+
±---------------------------------------------------------------------------------------+
| Grants for sstuser@localhost |
±---------------------------------------------------------------------------------------+
| GRANT RELOAD, PROCESS, LOCK TABLES, REPLICATION CLIENT ON . TO ‘sstuser’@‘localhost’ |
±---------------------------------------------------------------------------------------+

This seems to be similar to https://www.percona.com/forums/questions-discussions/percona-xtradb-cluster/49397-i-cannot-join-nodes-to-cluster-after-bootstrap and https://www.percona.com/forums/questions-discussions/percona-xtradb-cluster/49973-fresh-install-ubuntu1604-all-3-nodes-report-cluster-size-of-1 – neither of which has an answer.

This is interesting, from your second node it says it had found a saved state on boot:

2017-11-16T11:03:01.282066Z mysqld_safe Assigning 9df5f23e-cabc-11e7-b6e2-030d02b0e16b:0 to wsrep_start_position

Was the second node fresh install or was it part of the cluster before? Can you stop the second node, cleanup the data directory and start it again?