So I reached the last level today and completed it. I then tried for 3 stars and managed it on my 5-6th attempt. Here are some pointers on how to get 3 stars in Capital City, Downtown:
1. The most important tip: use tankers! There is a steady stream of tankers driving right to left on the upper roadway. Use the wheel glue or car controller to guide them into large buildings for massive damage.
2. There is a large fuel container in the middle of the map. Driving a tanker into it will instantly level the 7 surrounding buildings. Use the ramp to hit it. Take care not to get stuck between the parked cars or run over the guy standing there, because...
3. ...destroying all cars in the parking lot grants you three extra car controllers, as long as the pedestrian is alive. It might not be worth the trouble, but at least it's an option. You also get three extra controllers if you drive for 10 seconds, but you usually achieve this without even trying.
4. Use the earthquake bomb in the upper right corner of the map. A correctly placed bomb will destroy 7 buildings by itself. It should be possible to complete the level without the bomb, but it would be much more frustrating and infinitely harder. You can still get 3 stars if you use the bomb, so don't worry.
5. There are two fuel containers in the upper left corner of the map. Drive a tanker into those for a ton of extra cash. Remember to drive the parked tanker away first, straight ahead through the two buildings (a cow placed in between them should level both), and into the half-completed building.
6. If possible, visit the black cow market and purchase an increased multiplier ($80m) and bomb price reduction ($35m). These should make reaching the target score easier. I actually only had the multiplier increase when I got the 3 stars, but knocking those $300,000 off the price of the bomb should also help immensely.
7. The buildings in the lower part of the map can be a pain. Using tankers is the best way to quickly take out the leftmost two, as it shouldn't be too difficult to drive a tanker into them. The other cluster is best dispatched by using the bounce pad and wheel glue to launch a car into the largest building. The following chain reaction should level them all. Simply driving a car into any of them leaves the largest building standing half of the time.
8. There is also a beacon in the lower left corner of the map. Driving a car into it will detonate a structure near the upper edge of the map. This might be easier than trying to destroy the structure directly, but your mileage may vary.
9. Note that sometimes the tankers simply stop coming or the traffic from the upper roadway will dwindle to nothing. This can be incredibly frustrating and is likely a bug or something. I don't know what causes it, so if you suddenly no longer see tankers coming for a long period of time, they probably won't be coming any more and a mission restart might be in order.
10. Don't give up. That's really the best advice you can take. Once or twice I failed to complete the level with 1-2 buildings standing and another time I was less than 50k short of 3 stars. This can be terribly rage-inducing, but perseverance will pay off in the end.