Hello Raj1, thanks for reaching out to us! I would like to let you know that both network speeds are included on your plan (3G, 4G), the LTE Plus is also included when available and to answer your question, you do not have to pay extra if your phone gets connected to 3G.
You're more than welcome! If you have any additional questions or concerns, don't hesitate in letting us know.
Have a wonderful weekend.