We investigate the phase transitions in two-leg ladders systems in the incommensurate phase, for which the gap is destroyed by a magnetic field ($h_{c1}< h$) and the ladder is not yet totally saturated ($h < h_{c2}$). We compute quantitatively the correlation functions as a function of the magnetic field for an isolated strong coupling ladder $J_\perp \gg J_\parallel$ and use it to study the phase transition occuring in a three dimensional array of antiferromagnetically coupled ladders. The three dimensional ordering is in the universality class of Bose condensation of hard core bosons. We compute the critical temperature $T_c(h)$ as well as various physical quantities such as the NMR relaxations rate. $T_c$ has an unusual camel-like shape with a local minimum at $h=(h_{c1}+h_{c2})/2$ and behaves as $T_c \sim (h-h_{c1})^{2/3}$ for $h\sim h_{c1}$. We discuss the experimental consequences for compounds such as Cu_2(C_5H_{12}N_2)_2Cl_4

11 pages; some misprints corrected + one reference added; to appear in PRB