A third Brisbane team will most likely have less support than the Dolphins, it will certainly generate less hype. You are also adding 10+ more games at Suncorp which will just dilute the average crowd there. There will be more local derbies, so they become less of an event.
That's your diminishing returns.
This is just speculation presented as "fact".
Here's a few actual facts that counter your death riding of rugby league in Brisbane.
Local derbies generate higher attendances than games against interstate teams.
Redcliffe Dolphins and Brisbane Tigers will represent opposite sides of the metropolis.
Bulk of Dolphins fans will be from Moreton Bay. The attendance at Lang Park for the Dolphins vs Knights game dropped to 18,033 because the railway lines connecting Moreton Bay were down for maintenance.
Brisbane Tigers will generate the bulk of their support from south-eastern Brisbane, Logan and Ipswich. There's more people living in this area than Moreton Bay and it's closer to Lang Park.
In my opinion, Brisbane Tigers will have more supporters than the Dolphins.
How would more games at Lang Park dilute attendances?
It's not like Broncos fans are queuing up to attend Dolphin games. Nor vice versa. Each club has its own fanbase.
There's 1.6m people in southern Brisbane, Ipswich, Logan and Redlands. More than enough support for the Brisbane Tigers.