1.) 50 GB is acceptable for an OS only drive.
Question... Why split it into three instead of just running everything off the second partition??
2) 125 GB
3) 125 GB or what ever is left over.
Reply to your question:
By the way, is 50 gb for OS enough to include the system restore points, hibernation, future updates, etc.? For 4 yrs + w/o repartitioning the hard drive size..