Here the earliest text has the plural form were rather than the expected was (“nothing did he speak which were contrary to the commandments of God”). The 1837 edition, presumably as a result of Joseph Smith’s editing for that edition, made the change to the singular was. One could argue that the were occurs because of the implied plural in this negative construction (in other words, “all things he spoke were in accord with the commandments of God”). As discussed under Helaman 3:31, the original text sometimes uses were where was is expected after an existential there. In each case, the critical text will follow the earliest textual sources in choosing between was and were.
Summary: Restore the plural were in Helaman 8:3: “and nothing did he speak which were contrary to the commandments of God”.