unlikely they are trying to deceive you. as a hiring manager myself i have a difficult time preparing for and hosting interviews on top of my regular (and irregular) everyday business duties. likely they will reserve 2 or 3 days for a hiring 'blitz' where the hiring manager has dedicated those days to interview everyone back-to-back. though waiting three weeks is irritating it isn't unheard of.
I own and operate an engineering firm (under grad stanford engineering, certified millwright)
the hiring manager may be on vacation, or working on another project until then.