You must give your employer your Member Information Form on or before day 90 or you will automatically default to TRS Plan 3.
Whether you actively choose TRS Plan 2 or TRS Plan 3 or you default to TRS Plan 3 by not choosing a plan, you will not be able to change plans at a later date. The decision is irrevocable, so be sure that you have weighed all of your options carefully.
Several resources are available to help you evaluate your individual situation and make the choice that is best for you.