Given lines AO, BO, CO, you want to locate the positions of A, B, C on those lines so that they form the vertices of a triangle having those lines as medians. The size of the triangle is not determined by that condition, so you can fix A arbitrarily on the line AO. Construct the point D on the line AO, on the opposite side of O from A, so that
. Then D will lie on the side BC of the triangle.
The centroid O of the triangle is the centre of mass of three equal masses at the vertices. So the vertices A and C must be equidistant from the median BO. Construct a line AXY through A, perpendicular to BO, meeting BO at X, and such that AX = XY. Then construct a line through Y perpendicular to AXY (and therefore parallel to BOX). This line meets CO at the vertex C of the triangle, and thus determines the position of C.
Finally, you can construct B as the point where the line CD meets the median BOX.