  We observed strong superflares (defined as flares with energy in excess of
10^33 erg) on three late-M dwarfs: 2MASS J08315742+2042213 (hereafter
2M0831+2042; M7 V), 2MASS J08371832+2050349 (hereafter 2M0837+2050; M8 V) and
2MASS J08312608+2244586 (hereafter 2M0831+2244; M9 V). 2M0831+2042 and
2M0837+2050 are members of the young (~700 Myr) open cluster Praesepe. The
strong superflare on 2M0831+2042 has an equivalent duration (ED) of 13.7 hr and
an estimated energy of 1.3 X 10^35 erg. We observed five superflares on
2M0837+2050, on which the strongest superflare has an ED of 46.4 hr and an
estimated energy of 3.5 X 10^35 erg. This energy is larger by 2.7 orders of
magnitude than the largest flare observed on the older (7.6 Gyr) planet-hosting
M8 dwarf TRAPPIST-1. Furthermore, we also observed five superflares on
2M0831+2244 which is probably a field star. The estimated energy of the
strongest superflare on 2M0831+2244 is 6.1 X 10^34 erg. 2M0831+2042,
2M0837+2050 and 2MASS J0831+2244 have rotation periods of 0.556\pm0.002,
0.193\pm0.000 and 0.292\pm0.001 d respectively, which are measured by using K2
light curves. We compare the flares of younger targets with those of TRAPPIST-1
and discuss the possible impacts of such flares on planets in the habitable
zone of late-M dwarfs.
