Singular vs. plural when the subject has multiple objects in it

This is a question about deciding singular vs. plural verb where the subject contains multiple objects in it. Let me set the context first.

I have a mathematical problem where I need to find a solution to a problem. A single solution is a collection of three functions I need to find. For examples, the functions f(x) = 1, g(x) = 2x + 1 and h(x) = 3x + 2 may form a single solution. Similarly, the functions f(x) = x + 1, g(x) = 2x and h(x) = 0 may form yet another solution.

Now after solving the problem I find that there is only solution to the problem. Which of the following ways is the right way to express this thought?

1. The functions f(x) = 1, g(x) = 2x and h(x) = x + 2 is the only solution to the given problem.
2. The functions f(x) = 1, g(x) = 2x, h(x) = x + 2 is the only solution to the given problem.
3. The functions f(x) = 1, g(x) = 2x and h(x) = x + 2 are the only solutions to the given problem.

Option 3 sounds grammatically correct to me but sounds mathematically incorrect to me because the functions f(x), g(x) and h(x) are not three different solutions I have found to the problem. Those 3 functions together constitute a single solution to the given problem.

So is option 1 or option 2 correct? If none of the options are correct, what is a correct way to express this thought?