Will a 9-speed trigger work with an 8-speed RD
 

Considering upgrading trigger shifters to possible SRAM 3x9 double tap for straight bar but have an 8-speed rear SRAM X4 derailleur/gear setup. Will this work with the 9th gear being a phantom as far as the rear is concerned or must I stick to a 3x8 trigger shifter setup?

You will also have to change the rear cassette and chain to 9-speed ones, you can keep the derailleurs.

same brand, and similar series yes, no problem. However manufacturers change the derailleur pull ratio whenever they get bored so try to span too many years mismatch. This doesn't carry up to 10s because the cassette spacing changes between 9and 10s.