The average price of an apartment in Manhattan climbed to more than $1.3 million in the third quarter, according to the city’s top brokerage firms, which released their quarterly reports yesterday. The latest numbers are misleading, however, because they are based on sales negotiated between April and June, before the credit crunch that is roiling the national markets took hold of the economy. A more accurate picture will emerge only next quarter or the first quarter of next year.