As for your question about homing, I have a modified screen set, so the wording may be a bit different, but the basic idea is the same. Make sure you've got your steps per unit all figured out before you work on this, or your numbers will be wrong.
First, move your machine manually to somewhere in the middle, well away from any limits, and click reference all home, but be ready to hit stop. Note which direction each axis is moving. If any of them are moving away from the limit switch, click Config->homing & Limits, and check reversed, or home neg to correct it. Also make sure Auto Zero is enabled. Once you have all axes moving towards their respective limit switches, you can start setting up the limits.
Click reference all again, and let each axis hit the switches, but again, be near the e-stop button in case they don't stop like they should. If this all works as it should, this will tell mach3 where 0,0,0 is at, and will reset each machine coordinate to zero.
Next run each axis manually as far as you'd like it to move, and enter the number from the respective DRO into the homing and limits config page.
Last, soft limits must be enabled if you want the machine to actually use these constraints.
The setup is different for each machine depending on its configuration, wiring, LH/RH screws, etc. so you may have to fiddle with the settings in the homing & limits page to get things working as expected.
Hope all this helps
Z