ellipsis

[navi]

Could one say:
1-It would be good to talk to him.
instead of:
2-It would be good for you to talk to him.
or instead of:
3-It would be good for them to talk to him.

If the context makes it clear what to subject of 'to talk' is?

[Rusty]

Could one say:
1-It would be good to talk to him.
instead of:
2-It would be good for you to talk to him.
or instead of:
3-It would be good for them to talk to him.

If the context makes it clear what to subject of 'to talk' is?
The infinitive phrase, to talk to him is the subject of the sentence. It is "non-referential." It simply holds the place where the subject could go.
To talk to him would be good.

[danmahaffey]

To add to Rusty's excellent explanation of the inverted form of the sentence, let me say that to talk also needs a subject.

The subject of the infinitive does not have to be stated, but can be understood in context. When it is stated, and if it is a preposition, it must be in objective case.

So, It would be good to talk to him means It would be good for [someone] to talk to him. And context will tell you who the [someone] is.

When you see the sentence after Rusty's analysis, it looks like this: For [someone] to talk to him would be good.

Full sentence examples:
It would be good for them to talk to him.
For you to talk to him would be good.I hope this helps.

[navi]

Thanks Rusty and Dan and Merry Christmas to you and to everybody else.

So if someone says to me:
-John is worried about his weight.

can I say:
-It would be good to do some sports.
(Or: To do some sports would be good.)

instead of:
-It would be good for him to do some sports.
(Or: For him to do some sports would be good.)

[Rusty]

Thanks Rusty and Dan and Merry Christmas to you and to everybody else.

So if someone says to me:
-John is worried about his weight.

can I say:
-It would be good to do some sports.
(Or: To do some sports would be good.)

instead of:
-It would be good for him to do some sports.
(Or: For him to do some sports would be good.)
Thanks for picking up on 2. and 3. Dan. My answer applied mainly to 1.

Navi, you can say "It would be good to do some sports. Or: To do some sports would be good" if you and your listener both know you are discussing John. These sentences don't say who would be the beneficiary.
I think it is better to "point your finger" at "him."
It would be good for him to do some sports.
For him to do some sports would be good.
And better still:
To do some sports would be good for him.

[danmahaffey]

Or, do better than talk about John: talk to John. It would be good for you to do some sports.

Merry Christmas and Happy New Year to you, and to all our friends here.