The typesetter for the 1849 LDS edition accidentally omitted the indefinite article a before man. Obviously, in this context “the church of a man” is correct; note the preceding use of “the name of a man”. The 1852 LDS edition restored the indefinite article in this passage. Jesus is not talking here about some generalized “church of man” in distinction to the “church of God”.
Summary: Maintain in 3 Nephi 27:8 the two instances of “a man”, the earliest reading as well as the expected reading.