Can someone guide me to some clear documentation on what each of these 3 tools does?

redsn0w - Only Jailbreaking?
ultrasn0w - Only Unlocking?
blackra1n - both unlocking & Jailbreaking

My device has 3G 3.1.2 has only blackra1n and its unlocked + jailbroken and i'm not sure but i think hactivated also..

Just want to clear my basic concepts.

You were close, check below for a little more of a breakdown.

redsn0w - jailbreaking utility provided by DevTeam for all 3.0 + 3.1.2 devices (2G/3G/3GS/iPT) - recommended jailbreak utility if you're not using PwnageTool.

PwnageTool - custom firmware creator for jailbreak on all iPhone devices.

ultrasn0w - unlock utility provided by DevTeam for iPhone 3G running OS 3.0

blackra1n - jailbreak utility provided by geohot (and currently unsupported as he has left the iphone hacking scene again) for 3GS 3.0 to 3.1.2. Does not work on factory 3.1.3. Has been known to cause problems due to the lack of adding the afc2 service and lack of true push certificate assignment.

blacksn0w - unlock utility provided by geohot for 3GS 3.1 to 3.1.2 (and currently unsupported). Does not work on 3GS 3.1.3.