I perform a high precision measurement of the static quark-antiquark potential in three-dimensional ${\rm SU}(N)$ gauge theory with $N=2$ to $6$. The results are compared to the effective string theory for the QCD flux tube and I obtain continuum limit results for the string tension and the non-universal leading order boundary coefficient, including an extensive analysis of all types of systematic uncertainties. The magnitude of the boundary coefficient decreases with increasing $N$, but remains non-vanishing in the large-$N$ limit. I also test for the presence of possible contributions from rigidity or massive modes and compare the results for the string theory parameters to data for the excited states.