  We consider FRW cosmology in $f(R)= R+ \gamma R^2+\delta R^3$ modified
framework. The Palatini approach reduces its dynamics to the simple
generalization of Friedmann equation. Thus we study the dynamics in
two-dimensional phase space with some details. After reformulation of the model
in the Einstein frame, it reduces to the FRW cosmological model with a
homogeneous scalar field and vanishing kinetic energy term. This potential
determines the running cosmological constant term as a function of the Ricci
scalar. As a result we obtain the emergent dark energy parametrization from the
covariant theory. We study also singularities of the model and demonstrate that
in the Einstein frame some undesirable singularities disappear.
