I can help you with the .srt part but not the 3D part.
i used a subtitle tool on a clients machine a few weeks ago which could do various formats including .srt
I have just found a link here https://www.jubler.org/download/index.php
and discovered it is freeware with a suggested donation.
Best check the 3D part but if they have requested a .srt file then that is really just a specific text format layout that they would recreate the subtitles from so you may not need to do anymore.
Essentially its just a list of timecode ins and outs and the text required. So you can use the titles created by your editor as a guide.
best wishes
Neil
http://www.patience.tv