You have several issues to contend with here. A pair of shotgun barrels that are properly regulated for shot will typically cross fire with factory slug loads.

When I have run across drillings regulated for this sort of thing, the technique was to regulate the rifle barrel and the left shot barrel such that a slug would hit the same point as a bullet at slug range. The right shot barrel was then regulated with respect to the left barrel such that it would throw a shot pattern to the same point the left barrel did. This meant the right barrel was unregulated as far as slugs went.

If you can get eccentric choke tubes you may be able to come up with a set that will make any two barrels shoot together but all three will be a stretch.

I'd go for both shot barrels shooting a slug to the same point and never mind the rifle as I'd reserve that for long range shots. probably set the sights for the rifle barrel. In fact I'd set the sights for the rifle and then attempt to get each shot barrel to shoot to the sights and accept a little discrpency between them.

Meanwhile, if you are hand loading the slugs try a lighter powder charge and or slower powder while you are waiting for the heavier slugs to arrive.