Why doesn’t Article IV, Section 3 make Article I, Section 8 meaningless?******
Because they don’t conflict.
The properties which Art 4, Sec 3 refers to as fedgov owned can only be owned by the feds in accordance with the enclave clause. Said properties can only be owned by the feds for those specifically enumerated purposes stated in Art I, Sec 8 which have been stated on this forum ad nauseum.
Once those requirements are met then Art 4, Sec 3 is applicable.
Again, the Enclave Clause regards the District of Columbia. And the portion of Article IV, Section 3 that is making people struggle is, “...nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States....”