So you can hunt camels in Australia? Did not know that... Kangaroos either.

Out of the 3, I guess a few thoughts. The .243 bridges the gap well between the .22-250 and the .308, a little nicer on your smaller game. Complication: Having a .308win already, you might benefit from being able to make brass for either cartridge from the same brass if you reload. On the other hand, it's infinitely frustrating to get into the field and find out you have .243 ammo when you're holding a .308, or vice versa.

.270win is pretty similar to the .308win in overall power, a bit lighter weight bullet, but not significantly so on mid-sized (under 600lb) game. Doesn't count as a "gap bridger" in my book though.

6.5 Swede, same game as the .270win, high end power, not really a 'gap bridger', but better in my book.

Personally, I'd take the .270win off of the list, and between the swede and the .243win, I'd look at how much time I spend hunting big vs small game. I spend more time hunting small game, so I'd go for a .243, but if you spent more time shooting buffalo and deer (which I'm assuming your buffalo are pretty big), then I'd look at the 6.5swede.