I'm of the opinion that there can be more than three great Mahler orchestras in the world. To my ears, mind, and eyes, the best orchestras are those who respond the best to the "driver" driving them - the conductor. In the whirlwind of numerous worldwide concerts, recordings, etc., many of what we consider to be the world's great Mahler orchestras have had the good fortune to be conducted by what many of us consider to be the best conductors, variable though they may/have been. And yes, it's definitely a plus that these organizations have the best possible players in their employ. What if Mahler, Toscanini, Reiner, Walter, Beecham, and many others from the Golden Age had the opportunity to conduct a lesser orchestra in one of many lesser cities in which they appeared while at the height of their powers as opposed to earlier in their careers? Too bad there are few or no recordings in most instances to document this point. And to "put the shoe on the other foot", what about today's conductors who do not fit the "best" mode and have had the opportunity to conduct the best orchestras because they're considered superstars through being podium showmen rather than thoroughly prepared, experienced and consummate musicians (I won't name names)? In the latter instance, it will be the orchestra who plays on its own in spite of that conductor's inability to conduct an ideal performance.
There have been posts at another thread elsewhere in this forum addressing how good the Pittsburgh Symphony is. Prior to Reiner being music director of the Chicago Symphony and the Metropolitan Opera, he led the Pittsburgh orchestra for 11 years from 1938-49. And though most of the recordings he made with that orchestra have at least made it onto now hard-to-get CDs, the performance quality in some the recordings he made with that band approach ones of the same repertory he made later in Chicago. IMHO, his Ein Heldenleben with Pittsburgh is for all intents and purposes, practically as good as the one he did later in Chicago. Though he recorded a Lieder eines fahrenden Gesellen in Pittsburgh (his only Mahler recording there), unfortunately, there is no recording of it that he later made in Chicago, although we do have M4 and DLvdE.
After all, it was Mahler who said that "There are no bad orchestras - only bad conductors".
Wade