Update changelog

CHANGES.md

@@ -31,11 +31,20 @@ ones in. -->
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 ## __cylc-8.0rc3 (<span actions:bind='release-date'>Pending</span>)__
 
+Third Release Candidate for Cylc 8 suitable for acceptance testing.
+
 ### Fixes:
 
 [#4777](https://github.com/cylc/cylc-flow/pull/4777) -
 Reinstate the Cylc 7 template variables for xtriggers with deprecation warnings.
 
+[#4554](https://github.com/cylc/cylc-flow/pull/4554) - Fix incorrect
+implementation of the ISO 8601 recurrence format no. 1
+(`R<number>/<start-point>/<second-point>`)
+(see [metomi/isodatetime#45](https://github.com/metomi/isodatetime/issues/45)).
+This recurrence format was not mentioned in the Cylc documentation, so
+this is unlikely to affect you.
+
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 ## __cylc-8.0rc2 (<span actions:bind='release-date'>Released 2022-03-23</span>)__