In reply to nagar975:
count is not a rand variable, but all constraints must be met before calling post_randomize(). Since count is initially 0, it cannot meet the constraint that its value must be greater than 3.
You can write
constraint set_bit_cnt {(('d0 + arr[0] + arr[1] + arr[2] + arr[3] + arr[4] + arr[5] + arr[6] + arr[7] ) > 'd3 );}
The 'd0 + is make the addition result a 32-bit value. Other wise the sum is a 1 bit result.