The third heaven is the dome or firmament, which is a sea of glass. It's a rigid crystalline sphere on the edge of space which formed in response to the creation of gravity on the second day.
The Third Heaven
CSI: Hebrew
Searching for meaning in the original Hebrew presents a problem of the variety that makes us wonder if the Theologians of old had a clue, and, if they did, did they figure that they could fudge it because no one would notice?