adding a xb to my hbrtg tank ... is this a good idea ?

I like the second idea better, multiple arowana in one tank often leads to torn fins and missing scales. I'd rather have a perfect single specimen than three with torn up fins.

The problem with combining arowana is that with two you don't really have a pecking order, in most cases you end up with a dominant fish picking on the submissive fish. With three you have another fish to take some of the heat, and with four you get the aggression spread over even more fish. So yes three is certainly better than two, but five is better again. If you want to make a comm you're better off starting out with juvi fish all of the same size rather than trying to mix them as adults.