Yep, it's 3 x 3 x 3 = 27. However, that will max out at some point, a single wave entry in the incoming waves list can't be more than something like 2400 units or so.
To have ships hold fire, put them into low-power mode. I think the hotkey for that is K, but it's the little pause button when they are selected. That's particularly useful for cloaked ships, as you noted.