The summation of three monomials is known as a trinomial. Here, "tri" means "three". Trinomials contain three terms.

For example, x + 7y + 6z is a trinomial with three terms x, 7y and 6z.
x4y + 5z + 6 is a trinomial with three terms x4y, 5z and 6.
Trinomial Example

A Trinomial can be defined as a polynomial with only three terms. When operating with a trinomial, the following concepts are to be understood first:

Like Terms: These are terms with same variables.
For example: (2x, 7x), (8xy, -4xy), (2a2, -5a2)

Unlike Terms: These are terms with different variables.
For example: (4x, 8y), (3xy, 4x2), (5s,-6t)

Zero Property:
It is the combination of positive and negative like terms resulting in zero.

1x = x
-1x = -x
x0 = 1
x1 = x