ص، ز، س involve the tongue tip close to the lower front teeth, with the sound passing through the corresponding narrow space. They are called the ṣafiriyyah letters because of their characteristic hiss or whistle-like sound.
صَبْر — emphatic sad.
زَكَى — voiced zay.
سَمِعَ — voiceless sin.
Their points are closely related, but their qualities make them distinct. Do not replace sad with sin. A fuller treatment of their characteristics follows in the next chapter.