How to make a dropdown menu and 2 text fields required based on dropdown menu option of Yes?

You didn't specify the names of the fields involved, so I will give you a generic script that you will need to adjust with the actual field names. As the custom Validation script of the first drop-down you can use the following:

if (event.value=="Yes") {
	this.getField("Dropdown1").required = true;
	this.getField("Text1").required = true;
	this.getField("Text2").required = true;
} else {
	this.getField("Dropdown1").required = false;
	this.getField("Text1").required = false;
	this.getField("Text2").required = false;
}