  This paper establishes the $L^p$ boundedness of wave operators for linear
Schr\"odinger equations in $\mathbb{R}^3$ with time-dependent potentials. The
approach to the proof is based on new cancellation lemmas. As a typical
application based on this method, combined with Strichartz estimates is the
existence and scattering for nonlinear dispersive equations. For example, we
prove global existence and uniform boundedness in $L^{\infty}$, for a class of
Hartree nonlinear Schr\"odinger equations in $L^2(\mathbb{R}^3),$ allowing the
presence of solitons. We also prove the existence of free channel wave
operators in $L^p(\mathbb{R}^n)$ for $p>p_c(n)$, with $p_c(3)=6$.
