You should have plenty of time at dinner with a 3:30 ADR, however I would disagree that it is just a short walk from the Cirque venue. Unless you consider walking the entire length of downtown Disney a short walk. T-Rex is on one end of DTD and Cirque theater is all the way at the opposite end.
However if you have never seen DTD before, park on the West End side parking lot where the Cirque theater is so your car is there later (after dark) when the show is over. Then you can walk all the way through DTD to get to T-Rex seeing the sights and after you eat walk back through DTD back to the theater.
It may be a pain in the butt trying to park on the Marketplace side for your ADR then move your car to West side for the Cirque show. The DTD parking lot at that time of day on a weekend is almost impossible to find a parking space.
We went to Cirque on a Saturday but we got to the parking lot about 10:45 am (we went to Disney Quest at 11:30) and we were able to park right up front, but getting there after lunch or early afternoon might be different.
Give yourself 40 minutes to walk all the way down through DTD and 40 minutes to walk back (if you have small kids). It may not look far on a map but when you walk it, it feels like about a mile and a half at least.
But you don't have to walk all the way through the Marketplace side to get to T-Rex. It is not too far past walking through Pleasure Island. If you were going to Rainforest then you would need to go to the very tip of the Marketplace side.