I have the same problem. Plenty of tier 1 vip missions but rarely tier 2 missions and none tier 3 missions. But I found out yesterday that if you’re going into free flight and use the developer mode you can start a career mission from the tools menu. In that pop up window you can choose the type of missions. And the tier 3 vip missions are very rare. At least that’s what it says on that mission type.
But on the other side, I think the payment for those missions is way too low in general. Why bother doing those missions, when you can do medium cargo missions for at least 400000 up to over 700000 credits?
And to add to this, are these tier 3 missions depending on your rank? Or in general is the spawn rate of higher tier missions depending on rank?