In this work, we have successfully created several unprecedented discrete 3d-3d'-4f heterotrimetallic clusters in between lacunary polyoxometalates (POMs). By the three-step sequential introduction of metal cations into a trivacant lacunary POM TBA(4)H(6)[A-alpha-SiW9O34] (TBA = tetra-n-butylammonium) in organic media, five kinds of sandwich-type POMs with double diamond-shaped 3d-3d'-4f heptanuclear clusters (IIIFeM4Ln2, TBA(n)H(m)[FeM4{Ln(L)(2)}O-2(A-alpha-SiW9O34)(2)], M = Mn3+, Cu2+; Ln = Gd3+, Dy3+, Lu3+; L = acac (acetylacetonate), hfac (hexafluoroacetylacetonate)) were successfully synthesized for the first time. By introduction of two [Ln(L)(2)](+) units on the ends of pentanuclear clusters [FeMn4O18(OH)(2)](23-) and [FeCu4O28(OH)(2)](27-), the magnetic interactions between Mn3+ - Mn3+ and Cu2+ Cu2+ could be modulated. Among a series of the heterometallic heptanuclear compounds, IIIFeM4Lu2 exhibited the slow magnetic relaxation characteristic for a single-molecule magnet under the zero applied magnetic fields.