Parallel kinematic structures have a series of advantages that makes them adequate for the mini-robots and micro-robots construction: actuator positioning on the seating, miniaturisation, stiffness, positioning precision and repeatability, actuators separation from the workspace. For such reason, more and more parallel mechanisms with specified number and type of degrees of freedom have been proposed. In this paper is presented a comparative study of the geometrical models for two guided in

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... e points parallel mechanisms types having three degrees of freedom, so called parallel manipulators of "a" and respectively "b" type. These two mechanisms types differ only through the level on which the passive spherical joints are located. The complications which occur in the geometrical model are illustrated when the spherical joints are not located on the mobile plate level. Some simulation results within a complex simulation system, which has been developed, are presented at the end of the paper.The main contribution of this work is the achievement of a unitary approach regarding some classes of 3 d.o.f. parallel minimanipulators with triangular platform. The obtained results have shown that in the case of 'a" type mechanisms the expressions for the geometrical model and the implicit equations form is more simple than in the case of "b" type mechanisms.