Hi mamabea, poor you, sounds like you are really going through it at the moment, it really does wear you down doesn't it. Thankfully both of mine were good sleepers, but on the odd occasions when something went wrong, I found the sleep deprivation the pits, so my heart goes out to you.
Your neighbours sound very unreasonable. Try to ignore them, its not as if you are playing loud music or anything, big big difference to a crying child.
I'm thinking really hard about what could be causing your 3 yr old to wake. Do you think its just become a habit, maybe she's a bit jealous of the baby and doing it for attention? Whatever the reason, its so hard for you. I know it sounds a bit odd, but maybe she's over tired. My ds gave up his day time naps at about 2 but with hindsight I think he would have benefited from having them for much longer, I was always worried that if he slept too much during the day he wouldn't sleep at night. Worth a try maybe, just to see if it works. Maybe let her have a short nap during the day if that works in with your routine and, although its easier said than done, when the baby is napping, try to devote as much time to her as possible. You have a small age gap, similar to mine and its hard, hard work.
Good luck and I'm sure you will get plenty more advice on here.