You could set up a three step Bloodspiller combo if you like, but as it sounds like you've noticed, you would need to either increase the length on Delirium's buff or reduce the GCD while Delirium is active. You'd need a GCD length of 2.1s to get to your third Bloodspiller before the buff falls off (7 GCDs in 15 seconds). The reason why I suggested doing it with two steps is because 5 GCDs in 15 seconds requires no further alteration to Delirium. You also don't want to get to the point where the new combo replaces most of your standard combo uses.

This is fundamentally different from Gnashing in that your second combo can happen any time you could use Bloodspiller (i.e. any time you consume 50 blood or use Delirium). All that Delirium does is ensure that you can get three upgraded combos off in a row.