On standard way to compute scalar multiplication is to use Double-and-add algorithm:
The idea is to take the binary representation of your scalar $b = b_0 ... b_m$in your case $b = 3$ gives $b_0b_1 = 11$.
First you initialize your result $Q$ with $0$.
Then for each increasing bit index $i$, you set $Q = 2Q$ (computed with the doubling formula) and if $b_i = 1$ you set $Q = Q + P$ (computed with the addition formula).
In your example you have:
$Q = 0$
$Q = 2Q = 0$, $b_0 = 1$ then $Q = Q + P = P$
$Q = 2*Q = 2P$, $b_1 = 1$ then $Q = Q + P = 3P$
It is the generalization of user94293 comment. If your implementation is critical, be careful since the algorithm above is not time constant. You may prefer the Montgomery ladder in critical cases.