Is safe to upgrade from pg 9.3 to pg 10 directly using pg_upgrade or
is better upgrade, with pg_upgrade, from 9.3 -> 9.4 ->9.5 -> 9.6 -> 10.
Using pg_upgrade, it is definitely possible to upgrade 9.3 to 10 in one jump. We did this in production earlier this year for 1500 Postgres clusters.
At https://bricklen.github.io/2018-03-27-Postgres-10-upgrade/ I documented (in excrutiating detail, perhaps) how we upgraded those clusters from 9.3 to 10 with sub-15 minute downtime per cluster. The only real issues we ran into were some corrupted indexes that appeared to be related to 10.1 and 10.2. Upgrading to 10.3/10.4 and reindexing fixed those particular corrupted indexes.