Economists are raising 2018 growth projections after a strong second quarter, but disputes with U.S. trading partners, a fading boost from fiscal stimulus and rising short-term interest rates lead many to believe the boom won’t last much beyond that.

The average estimate for economic growth this year increased to 3%, up from projections of 2.9% last month and 2.4% a year ago, according to The Wall Street Journal’s monthly survey of private economists. They also see the unemployment rate falling to 3.6% by June, which would...