"a bit," "a little" and "a little bit"

[Daruma]

Hello.

This soup is too thick. Can you thin it down a bit?
This soup is too thick. Can you thin it down a little?
This soup is too thick. Can you thin it down a little bit?

Do these sound good?

Thank you.

[Ann1977]

Hello.

This soup is too thick. Can you thin it down a bit?
This soup is too thick. Can you thin it down a little?
This soup is too thick. Can you thin it down a little bit?

Do these sound good?

Thank you.

They sound perfect.

I think they are British ---> American in descending order, but only slightly so.

I would interchangeably say any one of the three in any context -- even within the same conversation.

"just a bit" recalls "just a tad," which is a little self-consciously or deliberately funny maybe, or light-hearted.

[PROESL]

Hello.

This soup is too thick. Can you thin it down a bit?
This soup is too thick. Can you thin it down a little?
This soup is too thick. Can you thin it down a little bit?

Do these sound good?

Thank you.

I've never thought of thinning down soup. I would say "thin out".

Other than that, yes, they sound good.

[Daruma]

Can you thin it down a bit?
Isn't it okay to use down here because "thin something down" means "to make a liquid less thick or strong by adding water or another substance"?

thin
verb
LIQUID
1 [vn] ~ sth (down) (with sth) to make a liquid less thick or strong by adding water or another substance: Thin the paint with water.

OF HAIR
2 to become less thick: a middle-aged man with thinning hair

BECOME LESS THICK
3 ~ (sth) (out) to become less thick or fewer in number; to make sth less thick or fewer, for example by removing some things or people: [v] The clouds thinned and the moon shone through. The crowd had thinned out and only a few people were left. Global warming could be accelerated due to the thinning of the ozone layer. [vn] Thin out the seedlings to about 10cm apart.

[Ann1977]

Can you thin it down a bit?
Isn't it okay to use down here because "thin something down" means "to make a liquid less thick or strong by adding water or another substance"?

thin
verb
LIQUID
1 [vn] ~ sth (down) (with sth) to make a liquid less thick or strong by adding water or another substance: Thin the paint with water.

OF HAIR
2 to become less thick: a middle-aged man with thinning hair

BECOME LESS THICK
3 ~ (sth) (out) to become less thick or fewer in number; to make sth less thick or fewer, for example by removing some things or people: [v] The clouds thinned and the moon shone through. The crowd had thinned out and only a few people were left. Global warming could be accelerated due to the thinning of the ozone layer. [vn] Thin out the seedlings to about 10cm apart.

I don't think anyone would notice "down."

I think I probably usually say "thin it out."