I just got to play a game with a bunch of unranked people in my team, 1 had gold - the enemy team has a guy who is rank 300 on the server. My support begins to feed him 3 kills in first 5 minutes of the game.
How can I practice in normal games when I get unranked teammates such as myself and we have to face diamond+ players? What's the logic behind it?

The logic is, those unranked players mmr is high enough
And the diamond player/rank 300 is low enough to
Be in the same game.

You’ll get more out of playing vs better players than you would smashing noobs while you “practise”

Or tl;dr get good.

Normals has their own mmr like mentioned above although, as you improve you will face better people, obviously it can be skewed by someone high queuing with low players in norms but also as mentioned above I wouldn't complain anyways it's how you improve. My norm queues on my flex acc are so long and are typically harder then my ranked games so I use them for practice as well or warm ups xd

the N is missing

Gringo