Even if other zoos at that time were not interested Taronga could of done a lot more than obtaining a pair and then down the track another one, its hardly going to help retain them other than cronic inbreeding, If a Asian zoo can import ten why could not Taronga import 5? and they would not be in the current situation they are facing now!
The decision of Taronga importing three instead of potentially five or six probably came down to the spatial reasons they would've experienced if they chose to import say six Bongo.
Yes, they're a Rainforest species but I agree with @Zoofan15 in saying they should be displayed at the Open Range zoos where they can be held in large groups, and which would've enabled a bigger import to be undertaken, similar to what Dubbo have done with their Black Rhinos in the past too.