Something is going on. The chances of THREE turbos leaking out of the oil seal is just out of this world. I would check oil pressure. Check the cooling system for oil. Check the oil for gas or coolant ect.. A blockage in the drain line would do this and you where on the right track when you checked it. The only odd thing is that the stock turbo works perfect. Thats the trump card in this.
I hate problems like this because you end up working to fix them for weeks. Like Steves car that ate the instruction manual. We find out after 2 weeks of messing with it that the cam timming is off.
Check that oil pressure Austin.. Your pump pressure relief valve could in fact be stuck.