How on earth (or whatever planet it is) do I win, or even survive, on level 3?

Frank's answer is the sneaky way. The brute-force way (and probably the one that the devs had in mind) is: wall off and fortify a choke point, then build a factory complex to churn out rockets and defender capsules. (And I suppose grenades if you like those; I wasn't impressed by them.) Everything you need is in your starting inventory, the trunk of your car, or the crashed ship you salvage at the beginning of the level. If you're careful, you won't need to build anything in hand.

I ran out of explosives and iron about the same time my turrets ran out of ammo. At that point, I had something like 70 rockets and 200 defender capsules, which is enough to pretty much just go Leeroy Jenkins on the biter hive. Keep 30-40 defenders out at all times, and keep moving, and not much will be able to hurt you; bombard the worms from long range with rockets, then move in and wipe everything out with your SMG and defenders.

I beat this one in 39 minutes. It was tedious, but what I did was slowly advanced forward with turrets defended by walls. Once I advanced into a new area, I would deconstruct the previous one and use that to move forward. Only thing you gotta build are repair kits. Wasn't sure if I'd make it through with enough bullets, was down to about 250 when the last nest was killed.

I went with about 30 defender capsule of which i had 10 retrieved from the first wreck. Make a small base with a few turrets until you get enough defenders. The important thing is getting near the shipwreck with the car. You have to close the distance as possible as u can with your car. Then go into the jungle. Try to make way near the shipwreck. If you are not attacked immediately, you wouldn't need to use the defender at first. I placed stonewalls behind me as i go, so the biters had to bite through it. I only used the defender when near the shipwreck and out of the jungle. Just make a quick run to it and grab it. It's also important to know the location of the shipwreck. It's besides the most southeast biter's camp.