Hmm, it's really too bad that this site doesn't have walkthroughs and strategy guides for games, that would make finding answers to questions so much easier and faster. (
http://www.gamebanshee.com/starwarskotorii/)
Well, since there's obviously no good information in these forums, as you'd never have asked if you hadn't already searched thoroughly, I guess we'll have to bemoan the lack of good resources. It's just such an unpopular game that no one bothers to actually write guides for it.
No point in checking GameFAQs, I'm sure none of the 20 FAQs and guides have anything useful (
Star Wars Knights of the Old Republic II: The Sith Lords Info - Star Wars Knights of the Old Republic II: The Sith Lords Information - Star Wars Knights of the Old Republic II: The Sith Lords Release Date).
Oh, well, I guess we just have to do this the hard way, and see if anyone on the forums knows the answer. Good news: someone does. Bad news: that someone is me, and you have to put up with sarcasm if you want to find out what it is.
The +3 CON bonus that you refer to is not a class bonus, it's an alignment bonus. You get a bonus if you are fully aligned with either the light or the dark. The form that bonus takes it dependant on your class. These bonuses do stack, so yes, you will get a +6 CON bonus as a pure LS Sentinel/ Watchmen, although this fact won't show up on your character sheet.
As to other stats, I'll quote from DSimpson's guide :
Throughout the course of the game, there are opportunities to get free increases to your Attributes:
+1 WIS - T3's final upgrade
+1 CON, +1 WIS - Hanharr's upgrades
+2 STR, +1 CON - Kreia's post-Hanharr upgrades
No mention of anything from Atton.
It's actually been years since I've played KotOR II, I got all of this for you with about 10 minutes spent browsing the GameBanshee and GameFAQs sites. If you follow the links above, I'm sure you'll find all sorts of useful information to help plan your character. If not, I'm sure we'll be hearing from you really soon.
(By the way, I do recognise that you're not a native English speaker, and I apologise if you've taken offence to this. But really, this game is over 2 years old and is based on one of the most popular franchises in the world. There are so many resources available for it that it's hardly necessary to ask this question once, rather than the 3 times you've asked in this thread. I do hope you read some of the FAQs I listed above, their author's make them freely available for your benefit.)