I'm not sure if the following works from a bash script, but at least I found out how to open specific tabs in specific directories:
In the Preferences, under Settings, you copy the profile that you would normally like to use. Under Shell, you can tell it to "Run command":
cd /some/directory. You will also want to check "Run inside shell". Optionally, you can specify a title under Window.
Add more profiles in this manner.
Then close all terminal windows, then open a profile in a new window, plus another profile in a new tab, plus another profile in a new tab. Now you have a window with three tabs. Save this as a window group.
From now on, whenever you have Terminal running, you can open this window group, and you will get a new window with these tabs in these directories.
Open question: how do I tell Terminal to open a new window group, from a bash script?