when do ㅅ and ㅊ and ㅆ sound like " t or d" ?

[kissthequizzer]

Because that has seriously been bothering me while I'm trying to get better at reading Hangul.

[manyakumi]

When they come to the end of a syllable.

낫, 낯, 났 all sound as 낟 when they were alone.
But when a vowel follows,

낫이 -> 나시
낯이 -> 나치
났어 -> 나써

Their sounds return.

[kissthequizzer]

does it always sound like a "ch" when it's at the end, too? like in 같 이 ?

[manyakumi]

does it always sound like a "ch" when it's at the end, too? like in 같 이 ?

It sounds like that only in front of "이 선행모음" such as 이,야,여,요,유.
And we call this phenomenon as "구개음화".

같이 -> 가치
굳이 -> 구지

같아 -> 가타
같은 -> 가튼