Equal intervals, called the common difference, separate the terms of an arithmetic sequence. The common difference can be either a positive or a negative number. For example, [2, 5, 8, 11...] is an arithmetic sequence with a common difference of 3, while [2, 1, 4, 7...] is an arithmetic sequence with a common difference of 3. In order to find a term in an arithmetic sequence, you need a starting point and the common difference. The general definition of the arithmetic sequence is a(n) = a(1) + (n  1)d.
Things You'll Need
 Calculator (optional)

Define the starting point. This is critical because all the terms you find have to be referenced to the same starting point. This will either be a part of the problem statement or you will have to choose it. Put the starting value in the arithmetic sequence equation.
a(1) = 3
a(n) = 3 + (n  1)d

Define the common difference. Like the starting point, this will either be part of the problem or you will have to choose it. Put it into the equation.
d = 5
a(n) = 3 + (n  1)5

Decide which terms you want to calculate. These will be the values for "n" in the equation. For this example, use n = [3, 7, 9, 11] for the third, seventh, ninth, and eleventh terms.

Calculate the value in the arithmetic sequence for each value of n.
a(3) = 3 + (3  1)5 =13
a(7) = 3 + (7  1)5 = 33
a(9) = 3 + (9  1)5 = 43
a(11) = 3 + (11  1)5 = 53 
Check your work, if practical, by writing out the series to include all of your values for "n." This will not be practical for very high values of n but it can increase confidence when using lower values of n.
a(n) = 3, 8, 13, 18, 23, 28, 33, 38, 43, 48, 53, 58
The third, seventh, ninth, and eleventh terms in this sequence match the calculations from the previous step.