What's the best way to record 3 bckground voices at the same time?  Should I record each 1 separately for greatest manageability, or set up 2 condensors on 2 separate ch.s on the board?

I would think recording them at the same time would work, but ideally in isolated fashion, so bleed would not be a factor.  You would not want one mic to pick up signal from a different vocalist.
 
I would want the ability to tweak each vocal line completely separately from any others, for EQ, pitch correction, etc., so I never choose to record them just standing next to each other, for example.
 
Bob Bone
 

More mics = more options. On the other hand, if you have a limited number of inputs you can record...
 
 

King Conga :  I do both for layering and the big sound ...Record everyone 1 at a time, your singers better be good ! this mix is center pan'd
                     Then record the group and use that ..pan'd to L-9pm   ..then record group again and use that pan'd to R-3pm... you now have a center mix as main drive and
                     A.... L&R  mix to fill things up, its very powerfull, I could tell you more but ....

I would make 4 recordings. One of each singer and then another of all 3 using a pair of mics using a mid side arrangement.
This will give you a nice thick layered sound.

If they're good, do at least one or two layers all at the same time. If they're iffy or you're not sure, separately. For layering it depends a lot on how "big" you want the result to be, and how capable the singers are. I've had up to 16 layers of one guy I think, but he's a real wizard at getting different tones that blend well. Three voices, left and right, different timbres... Do the math.

Here's a link on mid side mic settings courtesy of Universal Audio blogs: http://www.uaudio.com/blog/mid-side-mic-recording/

Also when you do the mid side recording of the 3 vocalists have them form an arc around the mics so they are equi-distant from each other but so the strongest singer is furthest from the mic while the weakest voice is closest. Once you have the singers standing in an arc move the mics so it is closest to the weaker voice and further from the strongest vocal till you achieve a balance between the 3 voices. The aim here is to capture each vocal at the same level within the mid side recording. This is important later when you mix all you vocals within the mix.

This will give you a total of 5 mic channels with ultimate control over the depth, spread and balance of the vocals as well as the tone and harmonies created by the interaction of the 3 individual and mid side recordings.

Hope that helps. There are so many ways to attack this type of recording. This is one way that has proved interesting to do and given good results as well.

It the vocalist are good and the room is OK (and the mic appropriate, etc. etc.), I'd let them sing naturally, cueing off each other and "premixing" as they sing.  You can, of course, take multiple takes.  I've had great success using two pzm's, back to back, with the singers in a rough circle around the mics so they can see each other.  An single omni should work, too.
 
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