With the scenario of multiple IP tunks for calling out, I have only a possibility to choose a script tl-dialout-2-trunks in the outbound route. How can I clone that script to allow more than 2 trunks for an outbound route?
well if you notice how its different than dialout 1 trunk, they both call the dialout base which runs a loop for each trunk you specify. Creating a dialout 3 trunk is as simple as repeating the part that changed (and sequence order) of adding the 2nd trunk.
Thirdlane/Asterisk Support available
However is it possible to modify the portal so that a third drop down shows up ?
you really should experiment before asking in the forum.. that answer is pretty self explanatory the minute you do the least bit of experimenting with scripts.