Okay, I re-read my post and realized I prolly confused you, forgive please and allow me to try again.
I have a nice bmp diagram (its 800kb tho)that I could send you, or if I can figure but how to get it up on a free web page, I'll come back here later and post that info to link but don't hold your breath cuz I'm not that internet savy.
Shut-off power (circuit breaker or fuse box) to the area.
A regular 2-way switch has 2 terminal screws and a ground screw(green). A 3-way switch has 3 terminal screws and a ground screw (green); 2 of the terminal screws are brass (or copper) and they are the "travelers" for between the two switches. The third terminal screw is usally black and that is called the "common" (for one of the switches it will be power IN from the house current, for the second 3-way switch it will be for the OUT of the switch loop and the IN to the the light fixture -- the light fixture will "neutral/return" this entire circuit to the house power circuitry).
First thing you determine is which switch location is the first point power is up from your house (that's the switch had has the HOT house (black)wire attached to the BLACK (common) screw . That's your "Switch A". (I suspect from your previous postings its the one with the most wires attached to it).
Then you need to trace (a little jiggle or a slight tug) at the wire from the switch while someone is looking at the wires in the box where the light fixture will be mounted) one of the wires attached to one of the brass screws (traveler #1 from "switch A") and see which one giggles in the box. Use some tape and in your light box this wire (traveler #1 from switch A), do the same for the wire attached to the second brass screw in switch A (traveler #2), and identify this wire end in your light fixture box and label it (traveler #2 from switch A).
Next, go to the second switch location (switch B) and note the wire attached to its black screw, that's your load wire or switch wire for your light, it could be any color (think from what you've said it might be red) but shouldn't white, but who knows what's in your house. That's the wire you tug and identify in your light box and lable as "Load (common) from switch B to light fixture". Then identify the wire attached to the left side brass screw on your switch B and have your partner mark in the light box "to traveler #1 switch B". Do the same for the other side brass terminal on your switch B and have your partner mark at the light location this wire as "to traveler #2 on switch B".
Now, there should be one remaining wire up in that light box that we haven't identified, and it should be white, so you return to the first switch box location (switch A) and have your partner jiggle that white wire, you should see it is attached to the neutral that originated in
your switch box. Have him mark that up at the light fixture location as "neutral return from light fixture.
Now....up in the light fixture box, identify the wires marked "traveler #1 from switch A" and "traveler #1 to switch B", twist them together and cap off with a wire-nut. Now identify the wires marked "traveler #2 from switch A" and "traveler #2 to switch B", twist them together and cap off with a wire-nut. Next, identify the wire marked "load (common) from switch B to light fixture" and twist it to the black wire of the light fixture, and cap off with a wire-nut. Then identify the wire marked "neutral return from light fixture" and twist it to the white wire of the light fixture, and cap it off with a wire nut. If your home system
requires or has ground wires....connect your ground wiring. keeping in mind the circuit I described.
Now lets get back to those other wires on your switch A, you have other switches in that same box, and check to see that the extra white and black wires you mentioned are
in some way wired to one of the other switches or to a jumper or pigtail. if this is the case, then I think we've got it. If you have a friend how's electrically savy and/or you can get your hands on a meter, now
is the time to check that things are as they "appeared" by our tug and pull investigation method.
If all is now well, Use electrical tape over your wirenuts, remove all those labels (they're fire hazards left in place), complete the installation of your light, re-install your switches to your boxes, mount your switch plates...and viola! you should now having properly wired your 3-way switches and completed a proper switch loop to your lighting fixture.
From your prior description I'm suspecting your switch A is the one that you mentioned had 1 Red, 1 white and 1 black and 1 black
back-stabbed; and that your switch B is the one that has 1 red and 2 blacks attached to it. Note the color of the screw terminals on each switch, 2 (on opposite sides) should be brass/copper screws and one should be black the ground screw is green, and usually below the switch box on the side.
I hope this was clearer.