Aspartic acid has $p{K}_a$ values of $1.88$ ($\alpha$-COOH), $3.60$ (side-chain COOH) and $9.60$ ($\alpha$-$\mathrm{NH}{}_3{}^{+}$). Taking pI as the mean of the two pKa values that flank the net-neutral species, pI is

With pH rising from low values: cation (+1), then net-neutral species after the first COOH ionises (pKa 1.88), then anion (-1) after the side chain ionises (pKa 3.60). The neutral species is flanked by 1.88 and 3.60, so pI = (1.88 + 3.60)/2 = 2.74. The pKa 9.60 is not involved.