Add changelog for 3.7.0-RC3

Author: WojciechMazur

changelogs/3.7.0-RC3.md

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+# Backported changes
+
+- Backport "Two fixes to NamedTuple pattern matching" to 3.7.0 [#22995](https://github.com/scala/scala3/pull/22995)
+- Backport "changes to scala.caps in preparation to make Capability stable" to 3.7.0 [#22967](https://github.com/scala/scala3/pull/22967)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.7.0-RC2..3.7.0-RC3` these are:
+
+```
+     4  Martin Odersky
+     2  Wojciech Mazur
+     1  Natsu Kagami
+```