$3 = (11)_2$
$3\times 4096 = 3\times (2^{12}) = (11)_2<< 12 = (11000000000000)_2$
Similarly, $15 \times 256 = (1111)_2 << 8 = (111100000000)_2$ and $5 \times 16 = (101)_2 << 4 = (1010000)_2$
So, $3\times 4096 + 15 \times 256 + 5\times 16 + 3 = (11111101010011)_2$
Number of 1's = 10.