Hi,
-3 is 1111 1111 1111 1111 1111 1111 1111 1101
0th bit is 1, 2nd bit is -4 which makes it (-4 +1= -3)
As << fills zeros from the least significant bit (R.H.S)
-3 << 2 is 1111 1111 1111 1111 1111 1111 1111 0100 which is -12
as 4th bit is -16 & 2nd bit is 4
i.e, -16 + 4 = 12.
Hope it helps.
KN