diff --git a/news/update-cpython-3-15-rc3.changed.md b/news/update-cpython-3-15-rc3.changed.md
new file mode 100644
index 0000000000..7462a425fe
--- /dev/null
+++ b/news/update-cpython-3-15-rc3.changed.md
@@ -0,0 +1,2 @@
+(toolchain) Updated default Python 3.15 runtime from `3.15.0rc2` to
+`3.15.0rc3`.
diff --git a/python/private/runtimes_manifest.txt b/python/private/runtimes_manifest.txt
index 8cb5df20cc..5bed4cc436 100755
--- a/python/private/runtimes_manifest.txt
+++ b/python/private/runtimes_manifest.txt
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 """
diff --git a/python/versions.bzl b/python/versions.bzl
index 18814e88b0..9b781242b8 100644
--- a/python/versions.bzl
+++ b/python/versions.bzl
@@ -57,7 +57,7 @@ MINOR_MAPPING = {
     "3.12": "3.12.13",
     "3.13": "3.13.13",
     "3.14": "3.14.4",
-    "3.15": "3.15.0rc2",
+    "3.15": "3.15.0rc3",
 }
 
 def _generate_platforms():
diff --git a/tests/support/mocks/python_ext.bzl b/tests/support/mocks/python_ext.bzl
index 87d492c538..7c1ac7bd36 100644
--- a/tests/support/mocks/python_ext.bzl
+++ b/tests/support/mocks/python_ext.bzl
@@ -111,7 +111,7 @@ _DEFAULT_RUNTIMES_MANIFEST = """
 9999999999999999999999999999999999999999999999999999999999999999  20241016/cpython-3.13.0+20241016-x86_64-unknown-linux-gnu-install_only.tar.gz
 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa  20260414/cpython-3.14.4+20260414-x86_64-unknown-linux-gnu-install_only.tar.gz
 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb  20251031/cpython-3.14.0+20251031-x86_64-unknown-linux-gnu-install_only.tar.gz
-cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc  20260929/cpython-3.15.0rc2+20260929-x86_64-unknown-linux-gnu-install_only.tar.gz
+cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc  20261003/cpython-3.15.0rc3+20261003-x86_64-unknown-linux-gnu-install_only.tar.gz
 dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd  20241205/cpython-3.13.1+20241205-x86_64-unknown-linux-gnu-install_only.tar.gz
 """