MAINLAND China firms are among those interested in leasing office space in the third and final phase of Central Plaza. China Empire Investment recently leased 20,000 square feet on the 63rd floor, and another Chinese company is negotiating a lease for 60,000 sq ft. Phase three, which comprises floors 47 to 74, is now 20 per cent leased. Of the 350,000 sq ft left, 170,000 sq ft is under offer or serious negotiation. Apart from the Chinese company, a major international company is also negotiating a lease for 60,000 sq ft. Floors below phase three are almost fully occupied. Insurance company Eagle Star has added a further 10,000 sq ft to its initial 19,000 sq ft. The Provisional Airport Authority, one of the earliest tenants, has taken up its option for a seventh floor. Motorola, Linkful Material Supply, NorAtel Communications and Mr David Lee, the project manager during the construction of Central Plaza, have all leased extra space. Two new developments within the building - a health club and restaurant - are due to open in the summer. Negotiations for the restaurant, which will occupy 8,000 sq ft on the harbour side of the 46th floor and be open to the public, are in the final stages. The executive health club, which will open on the third floor at the end of May, will be for the use of occupants of Central Plaza only. ''The take up rate for phase four is not as encouraging as it was for phases one and two,'' said Mr Chu Man Chung, deputy manager of Sun Hung Kai Properties. ''This is not because of the market but because of the higher rents.'' More than 100 companies are now in residence in Central Plaza, with 65 per cent of the 1.1 million sq ft of office space leased.