Principal often called initial amount of money, is the money that is borrowed, invested, or deposited. Interest Rate is always applied on the principal amount for calculating any form of interest rates.
Example:
if you take a loan of 500$ from a bank then 500$ is the principal amount.

Rate of Interest is the Percentage on which the Interest Amount is calculated.Genrally we represent rate of interest in Percentage Per Year. Rate is the main factor in determining how much interest we are going to earn or to pay to the bank.
Example:
If the rate of interest is 8% per year then it means the amount of interest will be 8$ for every 100$ which is invested or borrowed.

Time is the amount of duration the money is borrowed or invested. More the time more the interest amount will be. Generally we represent time in years while calculating the simple interest.