This quick guide shows you where all of the BFGs in vanilla Doom 3 are. Most of these shouldn't be hard to find, but a guide always helps.

In the middle of Delta2a, you will end up in an area where you need to use a yellow box to access a vent. After ending up in a vent, you'll be in a hallway with a door to your left. The door is locked, but can be opened by inputting 931 on the keypad to the right of the door. Go inside, then go to the wall that's the opposite of where you entered. There is a button that'll lower the glass on the nearby BFG. Hit it, then pick up the BFG nearby. This will also register an achievement.

At the end of Delta Labs 2b, you'll be above an office area. In front of your is the door to the exit (that's locked when you first get there), to your left is an abandonded office that has a PDA, and to your write is a door with a keypad. Go to the door on the right and punch in "725" on the keypad. This'll open the door.

When you're in, you'll see a storage locker. Go to it and put in the code "0508". It'll open and reveal a BFG.

At the middle of the level, you'll be in a dark grey room with pillars that's overlooking the sky and has two rounded rocks near the end of the area. Go between the rounded rocks, then go to the part that's facing the sky. You'll see a floating platform with a BFG on it. Pick it up. Beware that some Lost Souls will spawn, but the Machine Gun should make quick work of them.

This BFG will drop after killing the Sabboth (the cyborg guy). It's in front of him after he dies.

After going through the first, vagina-looking hall in the level, you'll end up in a room with a lava pit busting out of the base that has a Cacodemon hiding in it. Go past the lava pit and follow the path until you end up near a weapon rack. There is a BFG on the bottom of the rack.

A BFG is one of the weapons in a crate at the start of the level. Easy.