Fast $\gamma$-ray variability in blazars beyond redsift 3

High-redshift blazars are one of the most powerful monsters in the universe and $\gamma$-ray variability carries crucial information of their relativistic jets. In this work we present results of the first systematical temporal analysis of {\it Fermi}-LAT data of all known seven $\gamma$-ray blazars beyond redshift 3. Significant long-term $\gamma$-ray variability is found from five sources in monthly $\gamma$-ray light curves, in which three of them are reported for the first time. Furthermore, intraday $\gamma$-ray variations are detected from NVSS J053954$-$283956 and NVSS J080518$+$614423. Doubling variability timescale of the former source is limited as short as $\lesssim$ 1 hour (at the source frame). Together with variability amplitude over one order of magnitude, NVSS J053954$-$283956 is the most distant $\gamma$-ray flaring blazar so far. Meanwhile, intraday optical variability of NVSS J163547$+$362930 is found based on archival PTF/iPTF light curve. Benefited from multiwavelength activity of these sources, constraints of their Doppler factors as well as locations of $\gamma$-ray radiation region and indications for the SDSS high redshift jetted active galactic nuclei deficit are discussed.