Obviously, there are many, many truly fine recordings of the third symphony - a situation that simply didn't exist even 15 years ago. What I think is important, is establishing some criteria of what works and what doesn't work. I think it's OK for the two outer movements to go a bit slower than usual, as long as they're really well done (and that means everybody, not just brass and strings).
But I also think it's important that the inner movements not die along the way. There needs to be a continuous flow that connects all the highlights and bigger moments. I do think it's OK - even preferable - that the of the initial tempo of scherzo not go particularly fast. But the two long posthorn episodes need to keep flowing along - those episodes chew up a whole lot of time in the third movement (shouldn't sound TOO distant either). I also think it's somewhat important that in the vocal movement, the "hinauf ziehen" (pull up) marking be observed in the oboe part. This is not a make or break deal, but it does provide some badly needed color in what is, otherwise, a very subdued movement (albeit beautiful). One thing that is truly important, is that the short choral "bim-bam" provide plenty of contrast to the two movements that surround it. In other words, it needs to be truly bright and cheery, while its brief orchestral interlude gets down and dirty - bringing out the alternating salvos of the suspended cymbal and tam-tam (so often badly underplayed, and nearly non-existant on the Haitink/CSO M3).
The last movement need not go excessively slow as long as two things happen. First, the conductor must make it clear to the upper strings and celli, when there should be lots of vibrato, and when there should be much less vibrato. In other words, characterizing each little sub-section, yet connecting the overall line. The issue of portimenti - sliding the finger on the fingerboard between notes - needs to be organized and addressed as well. Second, whether fast or slow, the climax of the long brass choral has to have some real "umph" to it. The four trombones - divided into two part writing - musn't fall down on the job; they really have to keep blowing. Also, the bass drum roll should match the sound and intensity of the rolls eminating from the two timpanists. In so many recordings, you hear the timpani, but almost nothing of the bass drum. The effect should be that of a 727 roaring down the runway. I have to admit that this partciular passage is done exceedingly well on the recent Haitink/CSO M3. The problem is, it also comes across as too litte, too late (to me, anyway).
One recording that does an excellent job of keeping the inner movements flowing along, yet not rushing the two "happy" marches (in major) in the first movement at the same time, is the Simon Rattle M3. He also does an excellent job of charachterizing each individual movement. Unfortunately, the sound just isn't that great. Also, some feel that Rattle rushes the end of the symphony too much. Fortunately for me, that doesn't bother me. I'd rather that the ending of the sixth movement - with its alternating tonic/dominant notes in the timpani - go a bit fast than excessively slow. Chailly makes a smart play by progessively lengthening the final three chords (rather than making them equal length).