Changing html element with click event: how to restore default state of element using jQuery if/else statement
This is actually a pretty classic problem and I've addressed it in my code on numerous occasions. One of the simplest ways to do what you're doing is to just reset the state for all the elements and then correct it.

function txtResize() {
    var clicked = this.id;

    if ( clicked === "Text1" || clicked === "Text2" || clicked === "Text3" ) { //one of our text's were clicked
        $("#Text1, #Text2, #Text3").css("opacity", "0.2"); //reset them all to .2 opacity
        $(this).css("opacity", "1");//set this elements opacity to full
    } else { //its clearly not Text1/2/3
        $("#Text1, #Text2, #Text3").css("opacity", "1");  // return to default state
    }
 };


One nit-picky point is that we tend to prefer lower case in attributes for html. I would recommend text1, text2... or even better would be to give an id that better described what the element is on the page.