you could do something like this
float x = 3.;
vector pos = {0, 0, 0};
int numPoints = 3;
for(int i=0; i<numPoints; i++){
addpoint(0, pos);
pos.x += x;
pos.y += x;
pos.z += x;
}
but if you don't want to generate your points in a loop you could do something like this
float x = 3.;
int pt0 = addpoint(0, {0,0,0});
vector pos = point(0, "P", pt0);
pos.x += x;
pos.y += x;
pos.z += x;
int pt1 = addpoint(0, pos);