NEW YORK — The Manhattan office construction boom is over. Just three large office towers — of more than 500,000 square feet — are being built across New York City, with two expected to open in 2024 or 2025 and nothing else projected to go up for years. Normally, a handful of sites that size would be in various stages of construction, with at least one opening every year since 2018, according to JLL, a real estate services firm. Nearly 20 large office buildings that developers have proposed, inc