  The characteristic features of
Ca$_{10}$(Pt$_3$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ("10-3-8")
superconductor are relatively high anisotropy and a clear separation of
superconductivity and structural/magnetic transitions, which allows studying
the superconducting gap without complications due to the coexisting order
parameters. The London penetration depth, measured in underdoped single
crystals of 10-3-8 ($x =$ 0.028, 0.041, 0.042, and 0.097), shows behavior
remarkably similar to other Fe-based superconductors, exhibiting robust
power-law, $\Delta \lambda(T) = A T^n$. The exponent $n$ decreases from 2.36
($x =$ 0.097, close to optimal doping) to 1.7 ($x =$ 0.028, a heavily
underdoped composition), suggesting that the superconducting gap becomes more
anisotropic at the dome edge. A similar trend is found in low-anisotropy
superconductors based on BaFe$_2$As$_2$ ("122"), implying that it is an
intrinsic property of superconductivity in iron pnictides, unrelated to the
coexistence of magnetic order and superconductivity or the anisotropy of the
normal state. Overall this doping dependence is consistent with $s_{\pm}$
pairing competing with intra-band repulsion.
