I agree discoverability is not great; to be honest I can't remember how I discovered the plethora of commands I use, though I do have a habit of reading the help a decent amount. That's how I discovered "g?" (Rot13) :D
3dd won't work. You can use registers, though:
1. "ayy for the first line 2. "Ayy for the subsequent lines (repeat using .) 3. "ap to paste.
You can replace a/A with other letters to copy into multiple registers. If there are patterns (e.g. odd lines to A and even lines to B), you can even record a macro.
I think registers are not something most Vim users use, however. I have no data to back this up, but I wouldn't be surprised if Vim is like Git, where people learn the commands for 80% of scenarios and Google the rest. In Vim, case, they might even settle for a very inefficient solution if the problem is not recurring.