To solve this problem, we can set up an equation.

Let "x" be the number of dogs Mary has.

According to the given information, Mary has "3/4" of the number of dogs she has, which can be represented as (3/4)x.

She also has "3/4" of a dog, which can be represented as (3/4)(1) = 3/4.

So, we can set up the equation: (3/4)x + 3/4 = x.

To solve for x, we need to find the common denominator and then solve the equation.

Multiplying both sides of the equation by 4, we get: 3x + 3 = 4x.

Subtracting 3x from both sides of the equation, we get: 3 = x.

Therefore, Mary has x = 3 dogs.

So, the correct answer is:

C. 3