AMSTERDAM – The Dutch-based Sijthoff Group plans to build its third Holland Imax theater, this one an Imax 3-D 450-seater in Amsterdam.The group built Omniversion, the first Imax theater in Europe in the Hague 11 years ago, and Holland’s second Imax Waterstad (Watercity) Theater in Rotterdam five years ago. The $20 million Amsterdam Imax, which has yet to get a greenlight from city elders, is targeted for groundbreaking in late 1998 or early 1999. Sijthoff also is planning to build a new theme park at its Rotterdam site called the Magic of Life. The park will host rides and exhibitions with the human body as its main theme. For instance, one ride will duplicate what it would be like to sit inside the human brain.