The "man" of whom Paul spoke in the third person was himself (cf. 2 Corinthians 12:7-9). He referred to himself this way probably out of reluctance to speak of this matter. Moreover he wanted to minimize the effect of boasting, which citing such a spectacular experience would have produced. [source][source][source]
Paul could not tell (did not know) whether God had transported him physically into the third heaven (cf. Acts 8:39; 1 Thessalonians 4:17) or whether his experience had been a vision (cf. Genesis 15:12-21; Ezekiel 1:1). The third heaven probably represents the presence of God. It could be a technical description of God"s abode above the cloudy heavens overhead and beyond the farthest reaches of space that man can perceive. "Paradise" ( 2 Corinthians 12:4) is a good synonym for the third heaven (cf. Luke 23:43; Revelation 2:7). [source][source][source]
What Paul heard, not what he saw there, is that to which the apostle referred. That message was personal; Paul never revealed in Scripture what God told him. However, it had the effect of strengthening his faith and hope that the Lord would abundantly reward his sufferings. This experience evidently took place when Paul was ministering in and around Tarsus. He did so about A.D42 , 14years before A.D56 , the most probable date for the composition of2Corinthians. [source][source][source]
"The man who experienced the ineffable "ascent" even to the third heaven was the same man who had experienced the undistinguished "descent" from a window in the Damascus wall [1]." [2][source]