Stay away from using CTRL–C/V and your mouse in vim, or at least until you’re familiar with vim‘s way of text handling.
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SHIFT–V to enter visual line mode
Press 3j to go down 3 lines, or press j 3 times
y to yank/copy, x to cut, p to paste after cursor, P to paste before cursor.
Using hjkl will improve your workflow greatly as you don’t have to move your hands from the typing position to the arrows and the mouse.
There are a couple of ways to yank multiple lines without entering visual mode. One of which is to type <action><number><direction>. For example, y3j means to yank from your current row to 3 rows down.
If you want to yank the entire paragraph or sentence you’re in, type yip (yank IN paragraph) or yis (yank IN sentence) respectively. You can also do yi" (yank IN ") or ya" (yank AROUND ") to yank everything that’s surrounded by " on your current position.