Wasn't on last weekend's tour but I did the overnight there back in 2022 and the third floor was genuinely one of the more active areas we encountered. We had sustained footstep sounds tracked across about 15 metres of corridor with no one else up there, confirmed by two of us listening from different positions. The building does settle and there's a fair bit of debris on those floors so ruling out environmental causes takes patience, but what we heard had a clear rhythmic cadence that didn't match any thermal expansion pattern I've come across.
Would be really interested to know whereabouts on the floor you were standing when you heard it, and whether the sound seemed to originate from above or at floor level. That distinction matters quite a bit for working out what you're actually dealing with. Anyone else on that specific tour who can corroborate?