What is the 3rd screw on Tourney RD?

"B" tension. It controls the spring tension in the upper spring, and therefor how closely it tracks the sprockets. It should be 'tight' enough that the pulleys do not hit the next largest sprocket when shifting to an easier gear.

On a Tourney and other similar Shimano derailleurs set a 5-6mm 'b' gap between the largest cog and jockey wheel teeth. Screw clockwise to make the gap bigger and vice versa.

I used to set it with the chain off. I'd push the cage forward to roughly the same position as it would be with the chain on and in lowest gear to double check but that's not really necessary.

Using the relevant sized Allen key between the teeth will be the easiest way to set the gap accurately.

I should have just left it alone, I turned it out to see any difference and I didnt and I left it there but I got other things to worry about in the bicycle hobby.
It shifts just fine, just needs a little fine tuning.
What I noticed is when I click one click, sometimes it doesnt move the rear chain a gear. But I set the lower limit and the high limit perfect. So I got to play with the B-tension screw and the shifter turny knob.

matt4x4 said:

It shifts just fine, just needs a little fine tuning.
What I noticed is when I click one click, sometimes it doesnt move the rear chain a gear. But I set the lower limit and the high limit perfect.

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It's not shifting fine if it's not changing gear properly on each click. B tension will only make gear changes sluggish if the gap is way too big and you will have issues shifting into first gear/big cog if the gap is too small.

If the limit screws are correct then you may have another issue to sort out first before doing any set up/fine tuning.

Hanger alignment is always worth checking and a common cause of issues.

If hanger alignment is fine and you're derailleur is struggling to go to a smaller cog/higher gear then it's likely excessive shift cable friction so look at inner and outer cables for kinks, grime, fraying, etc or maybe just replace them with new ones, if you haven't already.

There's a list of things with drivetrains that could cause problems but everything has to be working properly in order for fine tuning to be effective.