You have only defined two columns. Column 0 takes its width from the size of the controls in that column, and column 1 takes up the remaining width of the grid.
If you want three columns, then you have to define three columns:
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
WPF Tutorial | Grid Panel[
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