Understanding cortical circuit computations requires recording methods with both unrestricted access to the entire cortical column as well as the ability to record from defined cell types in multiple cortical layers simultaneously. 3-photon excitation satisfies the first requirement but has only been applied to multi-layer acquisitions when multiplexed with 2-photon excitation by complex optical systems. We present a simple 3-photon imaging method which achieves simultaneous single-cell resolution imaging of two cortical planes at a maximum axial separation of 800 μm. This constitutes the largest known axial separation of simultaneously acquired 3-photon imaging planes at this resolution. With off-the-shelf lenses, we designed and constructed a relatively simple remote focus module based on a previously published non-unity magnification method. This is combined with a sensorless adaptive optics module positioned to correct for spherical aberrations caused by defocusing alongside other aberrations caused by scattering tissue. At all focal shifts between -350 and +450 μm, measurements of aberration-corrected z point spread functions were close to theoretically predicted values and indicate that the system is capable of single-cell resolution imaging over the entire axial range. To demonstrate proof of principle, we have performed in vivo imaging in Penk-Cre+ mice injected with AAV1-Syn-Flex-jGCaMP7f virus and recorded spontaneous activity from neurons in layers 2/3 and 6 simultaneously. This optical system is capable of simultaneously recording the activity of neurons residing in any combination of cortical layers and is therefore well suited to support new insights into interlaminar computations.