after you have created a 3D Text object you may want to trigger it via script. Here is how you can do that:
Obtaining a 3D Text Object for scripting
//add a public Text3D to your monobehaviour
public Text3D myText;
On the inspector , Drag the a 3D Text Object into the monobehavior myText field:

you can now use myText in your monobehaviour
Setting text and rich text via script
myText.SetText("My 3D Text"); // set a simple 3d Text using the current font settings
myText.SetText("My 3D Text",true); //set the text but do not animate
for rich text, you need to build a 3d text span. Add the following fields to your monobehaviour
public Font3DStyle firstFont; // the first font to set
public Font3DStyle secondFont; // the second font to set
now do this
var builder = new RichTextSpan.Builder();
//add each font and text.
builder.AddSpan(firstFont, 1.5f, firstFontText); // first font with size 1.5
builder.AddSpan(secondFont, 1f, secondFontText); // second font with size 1
var richText = builder.Build(); // call builder.Build() to get the richTextSpan
myText.SetText(richText); // set the text
You can find an example of calling SetText from script it Basics/Tutorials/Set Text and Basics/Tutorials/Rich Text