KBS Real Estate Investment Trust III Inc. has agreed to buy a 19-story office tower in Las Colinas for $46.3 million. The Dallas Morning News reported that the buidling is The Tower on Lake Carolyn at 909 Lake Carolyn Parkway in Irving. It was built in 1988 and has 364,366 square feet of space, the Morning News said. The building is 80 percent leased. The newspaper said that KBS announced its impending purchase in filings with the Securities and Exchange Commission.