I ordered an L3+ have been using it for a month. The difficulty is going up like crazy. By the time September batch starts shipping the machines wont be generating much. Besides that I have noticed this with Bitmain products for years now. And FYI Batch 1 of this miner was sold out in minutes. Another batch will be coming up soon. It says that one machine makes over $200 per day but by the time we get it it would make less than half of that.
Have you been noticing significant decrease in output from your L3+ over the month? Quite curious if that's what will happen to D3.
A rise in difficulty means that more miners have come online, adding to the total hashing power on the blockchain. This means that your payout will be less because you are competing with other miners. Yes, the same thing will happen to the D3. At what rate will this happen to DASH vs Litecoin? That depends on a lot of things of which are out of your control.