The stability of polarization, areas and number of self- induced transparency (SIT)-solitons on an output the LaF3: Pr3+ sample is theoretically researched versus of the polarization direction and the area of the input linearly polarized laser pulse. For this purpose the Vector Area Theorem is rederived taking into account directions of the dipole matrix element vectors of the six different subgroups of Pr3+ ions. It is shown that there are SIT solitons, which polarizations are directed so between as along the crystallographic axes in the LaF3:Pr3+. For an absorbing medium the interference of these ion's subgroups reduces that SIT-solitons are stable if its polarization is directed between the crystallographic axes. Solitons are nonstable, if its polarization are directed along one of the crystallographic axes: a small change of an input pulse polarization reduces to a soliton splitting up into two separate solitons, which polarizations are directed between the crystallographic axes under an angle +30 and accordingly -30 degrees to the given axis. For an amplifying medium it is just on the contrary.