Combine and trim files with silence

Hi All,

I'm using SoX v14.4.2 and would like help mixing three files, with 
silence in between two of them.

Problem I want to solve:
Mix three files with, with the longest being shorten to a specific file. 
I also want a 1.5 second pad between short_track.wav and main_file.wav

What I've tried:
sox -m main_file.wav long_track.wav -p pad 1.5 0 | sox - -m 
short_track.wav -t wavpcm out.wav trim 0 0 `soxi -D main_file.wav`

Problem is that the file cuts off about 1.5 seconds too quick, which is 
how long my pad is.


I've also tried creating a blank file:
sox -n -r 16000 -b 16 -c 1 silence.wav trim 0.0 1.5

Then combining it:
sox short_track.wav silence.wav main_file.wav long_track.wav out.wav 
trim 0 `soxi -D main_file.wav`

But you seasoned sox experts already know this means log_track.wav isn't 
used at all. This also cuts off about the last 1.5 seconds of the 
out.wav file as well.

So is it possible to combine these files and trim to the main file?

Any pointers would be greatly appreciated!