Regina's right - it's usually left out. It's a bit softer. This usually helps my students: begin saying the "t" in costly (make the shape/beginning sound with your mouth), but don't follow through with it. It's like you're "half" saying it, but you don't finish it.

I think that "quietly" is a bit different from the other two. The '"t" sound in the others is usually softer, or left out completely. In "quietly" it is usually heard clearly though.

Yes you do say it, but it is a little softer than when you say "t" in other words, like "eat" for example. Good pronunciation is key to communicating in a new language. It is important to be understood. Unless a letter is actually silent, ("lamb" for example) I would advise leaving out any letters to adhere to a particular accent. Hope this helps! :)

The 'T' sound is often left out in those words in American English accent.

depends on the specific dialect of american english. Usually its left out, but in some parts of america its not