Am I missing something simple?
You need the L flag (to prevent further processing) and this directive should go at the very top of your existing .htaccess file (ie. before the # BEGIN WordPress section).
Unless you have already defined an appropriate RewriteBase directive (the WordPress front-controller usually has this), then you also need to prefix the substitution string with a slash (although it is recommended to do this anyway for external redirects).
Otherwise, your regex and substitution string should work OK.
In other words, at the top of your file:
RewriteRule ^(.*)/item/(.*)-([0-9]+)$ /$1/item/$3 [R=302,L]
Test with 302 (temporary) redirects (to avoid caching issues) and change to a 301 only when you have confirmed it works as intended.
You will need to clear your browser cache before testing.