How long did you wait before you 'switched out the shrimp'? The key is to leave the shrimp in long enough to rot. With a 75 gallon tank and 1 shrimp, I would expect it to take over a week to cycle.
With 1 shrimp, it took me 5 days to get a spike on a 12 gallon tank, and I didn't even take it up to 1.0 like most people recommend.
Even with LR, you should see a spike, though you might not catch it with your testing.
I would throw the 3 shrimp back in, wait a week, test and retrieve. You should see something by then.