How can i remove three dots of excerpt?

You could do this:

.entry-summary {
    display: none;
}

To hide the button, you could give each post you want the button to display on a category. That way we can target the category and show the button, then hide it for the others.

Thank you for your reply.

I added css as you told but the buttons have also gone with 3 dots.

And I didn't understand what you said on buttons, can you please elaborate more.

For the posts you want the buttons, add a category (let's say button-posts) to the post itself, then we can use some CSS.

Let me know if this helps :)

How can i add that?

In the Edit Post screen, there should be a Categories metabox. Usually on the right hand side.