The answer to what's best depends very much on what the cartridge is exected to do. the 7.92 X33 was designed to be used for close to intermediate range ground combat in a large capacity magazine to reduce recoil and overall weight. The other two rounds were designed to be full size all range combat rifle rounds. It'a an apple and orange argument. Of the
.276 and 6.5 the .276 would have a slim advantage because of the .284 bullet diameter's better ballistic coefficient. The .276 would have been adopted except that it didn't make economic sense with the millions of rounds of .30-06 ammunition already in the supply system.
By .280 I'm assuming you mean the .280 round used in the postwar EM-2 rifles and not the much older .280 Ross another British round
"Introduced in the Mk 11 action in 1907 as a sporting round, the .280 Ross is a large semi-rimmed case, bigger and longer than the 7mm Remington Magnum that it preceded by over 60 years. The actual caliber of the projectile is .289"
The original factory loads were a pointed FMJ 180 grain target round at 2800 fps, and a 146 grain bronze point type spitzer hunting bullet at 3100 fps. This was in 1907 remember, before the word Magnum had been co-opted by the shooting fraternity, and without the advantage of modern slow burning powders. The .280 Ross is not far behind the performance of the excellent 7mm Remington Magnum."
A bit wordy but I hope this helps