WASHINGTON — Three mainline Protestant denominations — the United Methodist Church (UMC), the Presbyterian Church U.S.A. (PCUSA) and the Church of the Brethren — have experienced steady decreases in U.S. membership rolls, continuing long-term trends, according to separate June reports. A “state of the church” report issued by the UMC said its U.S. membership fell to 7.9 million — a…