We study the dynamics of SL3(R) and its subgroups on the homogeneous space X consisting of homothety classes of rank-2 discrete subgroups of R 3 . We focus on the case where the acting group is Zariski dense in either SL3(R) or SO(2, 1)(R). Using techniques of Benoist and Quint we prove that for a compactly supported probability measure µ on SL3(R) whose support generates a group which is Zariski dense in SL3(R), there exists a unique µ-stationary probability measure on X. When the Zariski closure is SO(2, 1)(R) we establish a certain dichotomy regarding stationary measures and discover a surprising phenomenon: The Poisson boundary can be embedded in X. The embedding is of algebraic nature and raises many natural open problems. Furthermore, motivating applications to questions in the geometry of numbers are discussed.proximally on R 3 , then P µ (Gr 2 (R 3 )) consists of a single element. We will refer to it as the Furstenberg measure of µ on Gr 2 (R 3 ) and denote it byν Gr 2 (R 3 ) .Remark 1.4. It will be important for us that the Furstenberg measure is atom free. This is ensured by the strong irreducibility assumption, since if there was an atom ofν Gr 2 (R 3 ) then the set of atoms with maximal weight is a finite Γ µ -invariant set.We fix {e 1 , e 2 , e 3 } the standard orthonormal basis of unit vectors in R 3 . For v ∈ R 3 and 1 ≤ i ≤ 3 we will write v i := v, e i . As before we consider the indefinite quadratic form Q :(1.1) Let H µ denote the Zariski closure of Γ µ . In what follows we will concentrate on two cases which will be referred to as Case I and Case II as follows: