We performed follow-up measurements of this object while it was still on the PCCP webpage.
Stacking of 50 unfiltered exposures, 30 seconds each, obtained remotely on 2021, January 27.1 from X02 (Telescope Live, Chile) through a 0.6-m f/6.5 astrograph + CCD, shows that this object is a comet with a compact coma about 10" arcsecond in diameter. (Observers E. Guido, M. Rocchetto, E. Bryssinck, M. Fulle, G. Milani, C. Nassef, G. Savini, A. Valvasori).
Our confirmation image (click on it for a bigger version; made with TYCHO software by D. Parrott).
M.P.E.C. 2021-C16, assigns the following orbital elements to comet C/2021 B3: T 2021 Mar. 11.18; e= 0.93; Peri. = 293.69; q = 2.15; Incl.= 119.50
by Ernesto Guido