Weed helps migraines, it helps tourettes (a little), but it will NOT cure, nor fix HIS three mental issues.. You have schizophrenia and bi-polar, which I can almost guarantee will get worse in time with repeated weed use as I have seen it NUMEROUS times in people.. maybe not so much the schizophrenia, but from 1 persons experience, and reading many things, I can say that smoking weed (in the long term) will make it way more apparent when you aren't smoking, and eventually since I'm guessing you smoke a lot already, you will develop a bigger tolerance (since you have more reason to smoke now) and your symptoms will start fading over to your high life.
The only one I'm not so sure about is the PTSD because it has varying results depending on the person, where it came from, how they use it to get over their problems..and how much they smoke. I know it's bad to say on a WEED forum that you shouldn't be smoking weed for those reasons, but I've seen it affect a gooood amount of people in a bad way after a while.. and why? Because negative effects from weed take a while to start getting to you when you have VERY SERIOUS mental health issues..
Now my question is, which ones were you diagnosed with first?
I'm purely not saying this to be the dick here, but from my PERSONAL experiences of smoking, and being around people who smoke for certain reasons, you start seeing a trend on what happens mentally to them, and I'm just trying to warn you as it always seems like a miracle cure for them before they find out too late (which usually ends in them quitting smoking weed forever)
But to also show that weed can be a GRREAATT temporary cure from personal experience (not relating to mental issues), it's helped me with my back problems which have gotten way worse this year due to either weight lifting or skating. I can go months without having a single problem and feel amazing, but the past few months I have been having severe back spasms which could either be from A: My trampoline incident that gave me HORRRIBLE back problems almost every week for a year, or B: My weight lifting accident that tore a major muscle in my back that made me stop doing any physical activity for a month. The spasms cause me to have my entire body tense up for 20 minutes - 3 hours leaving me almost paralyzed from pain.. Now it NEVER happens when I smoke, and if I smoke while it happens, the pain/spasms go completely away in a matter of seconds.
Anyways, good luck and I hope you are the SMALLLL percentage that it doesn't affect like the way I've seen it
Peace Not Greed.