I GUARANTEE you 1000% your b-dog is NOT actually getting a signal on 119 with a 3-LNB - rather, when you switch it to 119, I'm guessing does not have the intelligence to see that when it sends the 22k tone to switch to 119, it does NOT know it's actually NOT switching the LNB to 119. (the 3LNB ignores those commands, so it sends the same signal out either way...)
Rather, you probably are doing a good enough job peaking it on the 3 birds & getting the tilt/skew adjusted properly, so that if you DO put a 5LNB on the dish, it gets a good signal on 119 anyway.