For the font families combo:
<ComboBox Name="Families" ItemsSource="{x:Static Fonts.SystemFontFamilies}"/>
For the font styles:
<ComboBox Name="Styles">
<x:Static Member="FontStyles.Normal"/>
<x:Static Member="FontStyles.Italic"/>
<x:Static Member="FontStyles.Oblique"/>
</ComboBox>
And for the font weights:
<ComboBox Name="Weights">
<x:Static Member="FontWeights.Black"/>
<x:Static Member="FontWeights.Bold"/>
<x:Static Member="FontWeights.DemiBold"/>
<x:Static Member="FontWeights.ExtraBlack"/>
<x:Static Member="FontWeights.ExtraBold"/>
<x:Static Member="FontWeights.ExtraLight"/>
<x:Static Member="FontWeights.Heavy"/>
<x:Static Member="FontWeights.Light"/>
<x:Static Member="FontWeights.Medium"/>
<x:Static Member="FontWeights.Normal"/>
<x:Static Member="FontWeights.Regular"/>
<x:Static Member="FontWeights.SemiBold"/>
<x:Static Member="FontWeights.Thin"/>
<x:Static Member="FontWeights.UltraBlack"/>
<x:Static Member="FontWeights.UltraBold"/>
<x:Static Member="FontWeights.UltraLight"/>
</ComboBox>
And now to test:
<TextBlock
Text="This is some text."
FontFamily="{Binding ElementName=Families, Path=SelectedItem}"
FontStyle="{Binding ElementName=Styles, Path=SelectedItem}"
FontWeight="{Binding ElementName=Weights, Path=SelectedItem}"/>