diff --git a/ledsa/core/parser_arguments_run.py b/ledsa/core/parser_arguments_run.py
index 8e5fa95..27bb1b6 100644
--- a/ledsa/core/parser_arguments_run.py
+++ b/ledsa/core/parser_arguments_run.py
@@ -57,7 +57,7 @@ def run_data_extraction_arguments(args: argparse.Namespace) -> None:
     if len(channels) == 0:
         channels.append(0)
 
-    if args.red or args.green or args.blue or args.rgb and not args.step_3_fast:
+    if (args.red or args.green or args.blue or args.rgb) and not args.step_3_fast:
         args.step_3 = True
 
     if args.step_1 or args.step_2:
diff --git a/ledsa/tests/UnitTests/test_parser_arguments_run.py b/ledsa/tests/UnitTests/test_parser_arguments_run.py
new file mode 100644
index 0000000..de0726a
--- /dev/null
+++ b/ledsa/tests/UnitTests/test_parser_arguments_run.py
@@ -0,0 +1,34 @@
+"""Channel flags select the requested step 3 mode without a second analysis."""
+import argparse
+from unittest.mock import Mock
+
+import pytest
+
+from ledsa.core.parser_arguments_declaration import add_parser_arguments_data_extraction
+from ledsa.core import parser_arguments_run
+
+
+@pytest.mark.parametrize('flags, channels', [
+    (['-r'], [0]),
+    (['-g'], [1]),
+    (['-b'], [2]),
+    (['-rgb'], [0, 1, 2]),
+    (['-r', '-b'], [0, 2]),
+])
+@pytest.mark.parametrize('fast', [False, True])
+def test_channel_flags_run_step_three_once(tmp_path, monkeypatch, flags, channels, fast):
+    monkeypatch.chdir(tmp_path)
+    (tmp_path / 'config.ini').touch()
+    extractor = Mock()
+    monkeypatch.setattr(parser_arguments_run, 'DataExtractor', extractor)
+    parser = add_parser_arguments_data_extraction(argparse.ArgumentParser())
+    args = parser.parse_args(flags + (['-s3_fast'] if fast else []))
+
+    parser_arguments_run.run_data_extraction_arguments(args)
+
+    kwargs = {'build_experiment_infos': True, 'channels': channels}
+    if fast:
+        kwargs['fit_leds'] = False
+    extractor.assert_called_once_with(**kwargs)
+    extractor.return_value.setup_step3.assert_called_once_with()
+    extractor.return_value.process_image_data.assert_called_once_with()