K3Fe5F15 is a multiferroic material belonging to the K3M3IIM2IIIF15 family. Zero-field cooled and field cooled magnetization measured as a function of temperature demonstrate magnetic transition in K3Fe5F15, K3Fe3Cr2F15, and K3Cu3Fe2F15. Complementary to this, the magnetic behavior below the magnetic transition was studied via magnetic relaxation at different temperatures after switching magnetic field from H to -H. A slow change of magnetization on the hours time scale was observed and it was best described by a logarithmic time dependence for all three compounds over a broad temperature and field range. It follows that a distribution of magnetic moments over anisotropy barriers, which block the magnetic moments against reorientation, is present. We introduced a model of thermal activation of the magnetic moments of regions distributed over the barriers to describe the temperature and field dependence of the relaxation parameters. The dimensions of these magnetic regions were estimated to be of nanometer size.