We don’t write 3$ or 15$. The dollar sign must come BEFORE the number. The correct way to write this is $3 or $15. It’s the same with every unit of currency: ?3, ?15, ?3, ?15, ?3000, ?15,000, etc.
In English we use the comma for separating thousands from hundreds: 1,000 and 15,000 are examples. They mean one thousand and fifteen thousand.
We use the period (full stop) to separate whole numbers from decimals. For example, 10.5 means ten and five tenths, or ten and a half, and 12.45 means twelve and forty-five one-hundredths. When we write the figures for money, we use the period in the same way. So, $15.00 means fifteen dollars and no cents, and $1.45 means one dollar and forty-five cents.