I'll use the simplest way:
Money = $4000
Rate of Interest: 3% per year
So, how much does he get per year? Here: 4000 x (3/100) = $120 per year.
So how much does he get paid in 3 years? Here: 120x3 = $360 in 3 years.
Or you can say that he got $4,360 from $4,000 in a span of 3 years with 3% interest.
Hope I helped a bit.