But, I understand the utility of a zoom lens (I own several myself), and the new Pentax *ist (yes, that really is the name) is an interesting choice. As you note, it's also very light335 grams (about 12 ounces, body only). Pentax sells the camera in a kit with two Pentax zoom lenses, a 28-90mm and an 80-320mm, for about $600. That's a pretty good deal. But do the lenses compare to the Zeiss on the T3? In a word, no. They just can't, not for that kind of money. They're slow, fairly heavy, and probably contain, if not plastic, a fair chunk of lower-grade optical glass. I'd probably opt, at the least, for a Sigma 28-105mm f/2.8-4.0 Aspherical Autofocus lens. Weighs a little more than the camera body, so still quite a bit less than your current setup. Cost is about $200. For the longer zoom, you could try the Pentax 80-320mm for another $200.
Face it, adding zoom lenses adds weight, so there's no way to featherweight the deal. Unless you're a real serious shooter, I'd stick with your original homework on T3. A fabulous camera.