What is 3 cents in decimal form?

A decimal number is simply a way of representing a number in such a way that the place value of each digit is ten times that of the digit to its right. A decimal representation does not require a decimal point. So the required decimal representation is 3 cents, exactly as in the question.

Furthermore, a decimal representation does not imply a change in the measurement units. If this is required then you need to specify the units into which the quantity is to be converted.