  We report a study of $\Lambda_c^+ \to \Sigma^+ \pi^0$, $\Lambda_c^+ \to
\Sigma^+ \eta$, and $\Lambda_c^+ \to \Sigma^+ \eta'$ using the data sample
corresponding to an integrated luminosity of 980 $\rm fb^{-1}$ collected with
the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The
branching fractions relative to $\Lambda_c^+ \to \Sigma^+ \pi^0$ are measured
as: $\mathcal{B}_{\Lambda_c^+ \to \Sigma^+ \eta}/\mathcal{B}_{\Lambda_c^+ \to
\Sigma^+ \pi^0}=0.25 \pm 0.03 \pm 0.01$ and $\mathcal{B}_{\Lambda_c^+ \to
\Sigma^+ \eta'}/\mathcal{B}_{\Lambda_c^+ \to \Sigma^+ \pi^0}=0.33 \pm 0.06 \pm
0.02$. Using $\mathcal{B}_{\Lambda_c^+ \to \Sigma^+ \pi^0}=(1.25 \pm 0.10)\%$,
we obtain $\mathcal{B}_{\Lambda_c^+ \to \Sigma^+ \eta}=(3.14 \pm 0.35 \pm 0.11
\pm 0.25)\times10^{-3}$ and $\mathcal{B}_{\Lambda_c^+ \to \Sigma^+ \eta'}=(4.16
\pm 0.75 \pm 0.21 \pm 0.33)\times10^{-3}$. Here the uncertainties are
statistical, systematic, and from $\mathcal{B}_{\Lambda_c^+ \to \Sigma^+
\pi^0}$, respectively. The ratio of the branching fraction of $\Lambda_c^+ \to
\Sigma^+ \eta'$ with respect to that of $\Lambda_c^+ \to \Sigma^+ \eta$ is
measured to be $\mathcal{B}_{\Lambda_c^+ \to \Sigma^+
\eta'}/\mathcal{B}_{\Lambda_c^+ \to \Sigma^+ \eta}=1.34 \pm 0.28 \pm 0.06$. We
update the asymmetry parameter of $\Lambda_c^+ \to \Sigma^+ \pi^0$,
$\alpha_{\Sigma^+ \pi^0} = -0.48 \pm 0.02 \pm 0.02$, with a considerably
improved precision. The asymmetry parameters of $\Lambda_c^+ \to \Sigma^+ \eta$
and $\Lambda_c^+ \to \Sigma^+ \eta'$ are measured to be $\alpha_{\Sigma^+ \eta}
= -0.99 \pm 0.03 \pm 0.05$ and $\alpha_{\Sigma^+ \eta'} = -0.46 \pm 0.06 \pm
0.03$ for the first time.
