RelativeLayout:
<RelativeLayout HorizontalOptions="FillAndExpand">
<Button Text="Button 1" RelativeLayout.XConstraint="{ConstraintExpression
Type=RelativeToParent,Property=Width,Factor=.0000,Constant=0}"
RelativeLayout.WidthConstraint="{ConstraintExpression Type=RelativeToParent,
Property=Width,Factor=.3333,Constant=0}"/>
<Button Text="Button 222" RelativeLayout.XConstraint="{ConstraintExpression
Type=RelativeToParent,Property=Width,Factor=.3333,Constant=0}"
RelativeLayout.WidthConstraint="{ConstraintExpression Type=RelativeToParent,
Property=Width,Factor=.3333,Constant=0}"/>
<Button Text="Button 333333" RelativeLayout.XConstraint="{ConstraintExpression
Type=RelativeToParent,Property=Width,Factor=.6666,Constant=0}"
RelativeLayout.WidthConstraint="{ConstraintExpression Type=RelativeToParent,
Property=Width,Factor=.3333,Constant=0}"/>
</RelativeLayout>

StackLayout:
<StackLayout Orientation="Horizontal">
<Button x:Name="button1" Text="Button 1"/>
<Button Text="Button 222" WidthRequest="{Binding Path=Width, Source={x:Reference button1}}"/>
<Button Text="Button 333333" WidthRequest="{Binding Path=Width, Source={x:Reference button1}}"/>
</StackLayout>
