I, too, am confused by the OP's question.
My take on it is this, it sounds like you are talking about combining the textures, not just the uvmaps, so that it is one texture file and then cheaper to upload into SL.
In this case, how I would do it is:
First, open the texture files up in photoshop or gimp and look at their layouts. See, how best to arrange the texture first before doing anything else. The main thing is to use up blank spaces in the texture so you don't have to redraw anything or end up overlapping things on accident.
Next, open the uvmaps up in 3ds max using the UVW Unwrap modifier and select the entire map for each object and move it to the side (outside the main border in the middle). So, for example, you have 3 different maps. Move the first one up, the second one down and the third one right. This is so they don't overlap immediately when you combine the uvmaps.
Then, collapse all the unwraps to save your layout and attach the objects together. After that, add another UVW unwrap modifier and proceed to arrange them all into the center area without overlapping. It's best to start arranging them one object at a time to prevent confusion. I usually start with the largest object first as it'll take up the most space on a map.
Once done, go to Render UVW Template and open the template of the UVW map in your texture-editing program. Then you can proceed to, basically, copy-paste the parts of the 3 different textures together onto new mapping.
That should work if I understood your question correctly.