use_add_curve_operators.py

import bpy

def add_curve(location):
    bpy.ops.curve.primitive_bezier_curve_add()
    obj = bpy.context.object
    obj.location = location
    return obj

obj1 = add_curve((0,0,0))
obj2 = add_curve((0,1,3))
obj3 = add_curve((0,0,6))

# In case this code is run after a bunch of other code we want to 
# ensure the state of the scene
# :: deselect everything. no active either.
bpy.ops.object.select_all(action='DESELECT')

obj1.select = True
obj2.select = True
obj3.select = True

bpy.ops.object.join('INVOKE_DEFAULT')