I've never liked mixing flash units with studio lights. The recycle times are often off, and so you end up with inconsistent shots because the flash hasn't recycled when you take a shot too soon after the previous one. But if that's what you have, it will work, just be patient.
Do you have a radio flash trigger? Or a way to fire the SB-400 off-camera? If not, I think your best bet would be to put the SB400 on the camera, but it's very important that you put it into manual mode so that it doesn't use a pre-flash. The SB-400 will trigger the other two light via their built-in optical sensor.
Place one light to one side or the other. 45 degrees is a good starting point, but can certainly move it around as needed. This should be your main or key light. It's the one that should light up people's faces (and thus cause some shadow to show up on the opposite side of the faces).
Now...normally you might use the other AB light as a fill light...but you already have the SB-400 on the camera, which can play the roll of fill. This light should be less powerful than the key light, so that you can still see the shadows cased by the key light. There is no correct balance, it depends on your goals for the shot...but getting the balance just how you want it, is a skill that takes knowledge, time and practice...so I'd suggest you work at it.
To get you started, I'd suggest this:
-Camera in manual mode (pretty much necessary for using studio lights). Start with an aperture of F8, shutter speed of 1/125 and ISO 100.
-Set your SB-400 to it's lowest level, hopefully that is enough to trigger the key light.
-set up a test subject, a stuffed animal works well.
-Set up the key light (with umbrella or whatever you have for it). Maybe start at 1/4 or 1/2 power.
-Try some test shots and look for good exposure on the 'lit' side of the subject. To adjust the exposure, you can change the power setting on the flash, change the aperture or ISO. You can also move the light closer to or further from the subject. Check the exposure on the LCD but don't forget to use the histogram display as well.
-Once you have the key light exposing the lit side of your subject, you can start to play with the power on the fill light. The fill light will light up both the lit and the shadow side of your subject. So watch the shadow side until you get the shadow depth/detail that you want. This will also add light to the lit side, you you may want to back the key light off a little bit.
-Now, we still have the other B800...and since you are shooting with a background, I'd aim this light at the background. I use a short light stand so that I can hide a light behind the subject, but sometimes it does have to be to the side or above them.
-Adjust the power of the background light, to get the background exposure that you want. Only having one light for the background, you'll want to make sure you don't have any wrinkles in the backdrop.
This setup should work well enough for single subject or small groups, but if you're shooting larger groups, you may have to switch to a more flat lighting set up with both of the AB lights out in front of the subject. This adds the problem that you wouldn't have a light left for the background. You could try the SB400 for that purpose, but you'd need a way to fire it, as it doesn't have a built-in optical trigger. You could fire the AB lights with the supplied cord (if your camera has a PC socket), but being tethered to a cord can be an issue as well.
And if I haven't mentioned it, a tripod is a good idea because it keeps your camera position consistent.
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