If you want to somewhat improve situation of elves, I'd go with Celene+Briala. It gives Briala hard power - something B+G doesn't and it forces Orlais to at least pretend to some extent that they recognize Briala as their peer, at least as long as she manages to stay alive and in power... and she might not be all that easy to get rid of once she is actually allowed to rule something as a noble. She already has an almost-army and she seems pretty good in not dying, generally speaking... I'm not saying it will be easy or her success is guaranteed. More of the opposite, actually, seeing as it is not the last installment of the game and thngs like Dalish freaking minicountry potentially coming to existance in DA:O epilogue get dropped along the way and nobody references their existance ever. But, that's more of a metagaming thing - from what we learn in the game, C+B has some potential.
Briala+Gaspard on the other hand looks like a deal that is bound to backfire. Gaspard is a puppet through blackmail and blackmail material like that gets old after a couple years. Briala has no hard power - just that coming from the blackmail - so when Gaspard manages to off her (that might take him years but I believe he will make it, since in his situation getting rid of her is his primary goal if he wants to break free and actually rule anything) elves have nothing again. Especially as with all that talk about honor, I can see Gaspard taking vengeance on elves for his humiliation. Elves in the Orlais won't gain anything in the long run and can lose much - that's how I see it. With B+G any good long-term scenario seems extremely implausible.
Finally, there is the "threesome" ending, that's just a forced truce, definitely not a stable situation. However, it's not necessarily bad - as long as they are forced to work together, they may be actually pretty effective. When they start to break free from your grasp, they have good chances of tearing Orlais apart... But (from elven perspective which you seem to hold dear seeing as you want "Briala in" scenarios) the reforms made during this first period have a good chance to live longer than the triumvirate. Those won't be big changes, but they probably have the least potential to backfire on the elves too, that's what I think at least.