When to use UserControl vs. Control in Silverlight?

If you consider your control to be a group of three text boxes then a UserControl would be appropriate, but if your control will still essentially be a TextBox then you should extend the existing control with "Make into control."

It sounds like you need a UserControl to me.

UserControls are meant to be a composite control - basically a bunch of other "controls" grouped together to work as a single, cohesive unit.

Custom Controls, on the other hand, are intended to be used as a single control. Think of the basic controls in the framework, such as TextBox or Button - if you were implementing something like that, you'd want a Control. (This is less common than UserControls, especially in WPF, since you can use templating on base class controls to accomplish quite a few things where you'd need custom controls in other frameworks). A custom Control is all about defining new behavior for a single "control."