Two magnetic transitions are observed in Fe3BO5, at 112 K and 75 K : each one corresponding to the independent magnetic ordering of ladder like sublattices, 3LL1 and 3LL2. 3LL1 orders at 112K; between 112 K and 75 K, spins on 3LL2 remain idle, and order only below 75 K.In view of this intriguing behaviour, the Fe3-xMnxBO5 solid solution has been investigated. Most surprisingly, for Fe1.5Mn1.5BO5, only one magnetic transition, coupling both 3LLs, at TN ~ 100 K is observed. An explanation that can be proposed to understand the decoupling of the sublattices in Fe3BO5 is the fact that magnetic exchange along c is antiferromagnetic in 3LL1, and ferromagnetic along c in 3LL2, thus preventing the coupling of both. In order to verify this hypothesis, we would like to investigate the excitation spectrum of Fe3BO5 and Fe1.5Mn1.5BO5.