OG Reading?

Doing my first batch on Saturday. I am a NOOBIE...When do you take the OG reading? I am going to boil 3 gallons and then top off with bottled water in the primary. Do I just take the reading in the wort before it goes into the primary or after all is in the primary?
Sorry if this is a repeat post...couldnt find an answer when I did a search..
Thanks

Check the gravity after everything is in the primary EXCEPT the yeast. So, top off with water, aerate, check the temp to make sure it's below 80*, check the gravity (may have to wait until some of the bubbles have gone away, pitch the yeast, cover and wait 3 weeks.

Your OG will let you know if your beer is on target. Don’t do what monty said, take a reading at the end of your planned boil time. Put the sample in some sort of container that will float your hydrometer and not waste too much wort. Chill the sample to 60* F, there are corrections for hotter/colder wort so you don't have to waste too much time chilling the wort. If the reading is lower than your target gravity you need to boil more and repeat the process. If the reading is higher then you need to top off with water and take another sample until your gravity is on mark.

When I first started I did what monty did and a couple beer lacked the ABV I was going for. Unless your practice is perfect I would recommend doing it this way.... I would recomend the same process for you monty

tobrew - You do have to top off with water to your fermenting volume before you take your OG. Otherwise it will be condensed and not a true reading. Do what monty3777 recommended. Sample just before pitching the yeast.

well I believe you guys are talking about extract brewing, my bad, but if you go all grain and you want to nail your target gravity the methods I listed will help you do so

Won't adding two more gallons of water (as the OP indicated) after the process you suggest change the OG?

well yes, and you would add extra water if you OG was too high. Again sorry to confuse anyone by my post, I was talking about all grain brewing...

tobrew, definitely take the reading after you've mixed in your top up water.

good luck on your first batch!