You should use a DataTrigger with MultiBinding -
<MultiBinding Converter="{StaticResource myConverter}">
<Binding ElementName="textBlock1" Path="IsMouseOver" />
<Binding ElementName="textBlock2" Path="IsMouseOver" />
<Binding ElementName="textBlock3" Path="IsMouseOver" />
</MultiBinding>
and in the converter if one of them is true, return true.
Another option (the first one is better) is to have 3 DataTriggers for IsMouseOver any of the textblocks (give them names, and use ElementName in the Binding).
<DataTrigger Binding="{Binding ElementName=textBlock1, Path=IsMouseOver}" Value="True" >
<Setter Property="TextDecorations" Value="Underline" />
</DataTrigger>
<DataTrigger Binding="{Binding ElementName=textBlock2, Path=IsMouseOver}" Value="True" >
<Setter Property="TextDecorations" Value="Underline" />
</DataTrigger>
<DataTrigger Binding="{Binding ElementName=textBlock3, Path=IsMouseOver}" Value="True" >
<Setter Property="TextDecorations" Value="Underline" />
</DataTrigger>