There are two ways to do this.
First way use a grid with ColumnDefinitions showing the width of each element Example (ColumnDefintions="2*,,*2) and then define each element and provide the Column number (starting with zero) for each of the three elements.
Second use a HorizontalStackLayout and add each of the three elements to the HorizontalStackLayout. Then in each of the elements provide a WidthRequest.
Personally I would use the 1st way as using the relative widths means that it will handle different sizes of the device on which the app is running.