When a growth interruption is applied to the top interface of a nominally 3 monolayers (ML) thick InAs quantum well (QW), grown on an InP substrate, the InAs layer relaxes by formation of three-dimensional (3D) islands. The emission from the QW is split into at least 8 peaks. Each peak corresponds to an integral number of MLs of InAs. Using cathodoluminescence imaging and photoluminescence excitation spectroscopy, we demonstrate that the emission originates in 3D islands with a lateral extension of less than 1 mu m. Furthermore, most of the 3D islands are completely isolated, separated by InP, even though there are 3D islands of different thicknesses that are connected. A conclusion from our investigation is that the whole InAs layer, rather than just the top ML, is involved in the 3D island formation.