ث shares the bowl shape with ب, پ, ت, ٹ. The only thing that distinguishes these five is the count and position of dots. Have your child trace ث right after ت so the difference is felt: two dots becomes three. The stroke timing is approximate; the rule that matters is bowl first, then dots, counted out loud.

Counting matters here. When your child gets all three dots in place, name it: "You counted to three and put each one above. That is se." The specificity is what they will remember.