Scalar multiplication with projective coordinates I'm implementing point addition, doubling and scalar multiplication using projective coordinates. I took reference from this link https://www.nsa.gov/ia/_files/nist-routines.pdf 

I have implemented 

Routine 2.2.6(ec_double)

Routine 2.2.7(ec_add)

Routine 2.2.8(ec_full_add)

Routine 2.2.9(ecc_full_sub)

They all are working and I have verified the results with the example results given at the last for P-256 curve.

I implemented Routine 2.2.10 ec_mult(Scalar multiplication) But I couldnt get the correct output. In line 15 and 16 of this routine they take the binary representation of d(random number) and 3d. And the loop skips both MSB and LSB and performs the operations.

I got doubts like that 3d is modular multiplication or simple multiplication?. The loops runs for d or 3d if its not modular multiplication?

Is there anything else I'm missing out?. If somebody has followed this algorithm and implemented it please help me out here.
3d multiplication is a simple multiplication not a mod multiplication.

I suggest you to check IEEE Std 1363-2000 document and "A.10.3 Elliptic scalar multiplication" part of that document if you can.
It has somewhat more explanation than Nist's document.