To determine the number of solutions of a system of linear equations, we need to analyze the coefficients of the variables in the equations.

Given system of equations:
- x + 5y = - 1
x - y = 2
3y = 3

Analyzing the first equation:
The coefficient of x is -1, and the coefficient of y is 5. Since both coefficients are non-zero, this equation represents a line.

Analyzing the second equation:
The coefficient of x is 1, and the coefficient of y is -1. Since both coefficients are non-zero, this equation also represents a line.

Analyzing the third equation:
The coefficient of y is 3. Since the coefficient is non-zero, this equation represents a line.

Now, let's consider the possible cases for the number of solutions:

1. Infinitely many solutions:
If the three lines are coincident (i.e., they lie on top of each other), the system of equations has infinitely many solutions. In this case, the equations are dependent.

2. Two distinct solutions:
If the three lines intersect at two distinct points, the system of equations has two distinct solutions. In this case, the equations are independent.

3. Unique solution:
If the three lines intersect at a single point, the system of equations has a unique solution. In this case, the equations are independent.

4. No solution:
If the three lines are parallel and do not intersect, the system of equations has no solution. In this case, the equations are inconsistent.

Analyzing the given system of equations:
From the coefficients, it is clear that the three lines are not coincident, parallel, or coincident. Therefore, the only possibility is that they intersect at a single point, implying a unique solution.

Hence, the correct answer is option 'C': Unique solution.