Well, just keep trying for the zero missions. It will take a long time, but you should eventually beat the mission. Try to shoot down the planes that come in large groups all at once, I found that helped me just beat the mission with seconds to spare.
Lots of people find the second mission to be even more difficult though. All I can say for that mission is take out the close couriers first, then go after the north ones. And don't take more than two passes at a courier, otherwise you'll spend too much fuel.
The third mission is very easy though. I would have beat it on my first try if it didn't take me 5 minutes to find out that the circle button works the magnet...
But, once you beat the missions, you get the RC Shop as an asset which earns a very respectable $5,000 a day.