PHILADELPHIA – An affiliate of Denver-based Amstar Group Ltd. has purchased the 20-story, 588,285 sq. ft. Three Parkway office building in Center City Philadelphia. The purchase price was not disclosed. Three Parkway, which underwent $28.7 million in renovations in 1996 and 1997, was the national headquarters of Reliance Insurance Co. The Class-A building features 557,440 sq. ft. of office space, 18,029 sq. ft. of retail space and a 12,816 sq. ft. basement that offers a fitness center and a production studio. There are 360,000 sq. ft. of space available for.