Can you imagine being that hopeful about the future, I know a lot of people are opting not to have children due to the environmental nightmare those children would have to survive.
Even if climate change is as bad as WW2 (doubtful over the same time scale), people still had children then, and they still had children when every city was covered in a foot of horse poop, all your kids died at 14 and doctors didn't believe in handwashing. In fact they had more!
It takes a bit less than 3x resources to have 3 kids though, since they share things.