Add changelog for 3.1.3-RC3

Author: Kordyjan

changelogs/3.1.3-RC3.md

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+# Notable changes
+
+- More aggressive reduction of type selection (fixes parboiled2) [#14987](https://github.com/lampepfl/dotty/pull/14987)
+- Relax avoidance checks more for match type reduction [#15036](https://github.com/lampepfl/dotty/pull/15036)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.1.3-RC2..3.1.3-RC3` these are:
+
+```
+ 2  Guillaume Martres
+ 2  Paweł Marks
+```