I am finding in my script 3Dbox, that only the first image created gets its virtual-pixel set to none. The subsequent images revert to edge. This does not happen in IM 6.7.6.10 nor 6.7.4.10
Input Images:



Here are the commands taken from my script:
s1="0,0 0,24.2195 127,0 99.3534,-49.6767 127,127 81.927,40.9635 0,127 0,112.896"
proc1='mandril.jpg -matte -filter point +distort Perspective "$s1"'
s2="0,0 -99.3534,-49.6767 127,0 0,24.2195 127,127 0,112.896 0,127 -81.927,40.9635"
proc2='logo3.gif -matte -filter point +distort Perspective "$s2"'
s3="0,0 -99.3534,-49.6767 127,0 0,-104.25 127,127 99.3534,-49.6767 0,127 0,24.2195"
proc3='zelda3.jpg -matte -filter point +distort Perspective "$s3"'
eval 'convert -virtual-pixel transparent -mattecolor none \
\( '"$proc1"' -write 1tmp1.png \) \
\( '"$proc2"' -write 1tmp2.png \) \
\( '"$proc3"' -write 1tmp3.png \) \
-background black -layers merge +repage \
1tmp_im.png'
Here is the result from IM 6.7.8.6

You can see from the -write images above that only the first image gets -virtual-pixel none and the other two get -virtual-pixel edge
Here is the result from IM 6.7.4.10 or IM 6.7.6.10

P.S. I see now that the order of the commands is not proper IM 6 syntax and I have fix the script by moving "-virtual-pixel transparent -mattecolor none" into each of the $procX variables.