I think A is actually not necessarily a triangle if it is to be a form. What ever A is, it is a label which in some way encapsulates the set of all conforming objects. Taking this into account, A cannot just be a triangle, since A was initially chosen to be a form representing a multiplicity of triangles based on certain characteristics. If A is one of those triangles then it is not a form, since it has other properties (like side length, which you mentioned) which the objects conforming to it do not have.
We simply cannot directly put an ideal on equal footing with the objects it represents. I can go into detail about how mathematicians have attempted overcome this issue if you'd like; an ideal of a ring is actually one construction that comes to mind.
Edit: After rereading your post, it seems that you are not concerned with self-referential objects; i.e., letting A conform to A. However, as in all instances of self-referentials, there is no way to define A and not get a contradiction.
[This post assumes that you know what the third man argument is]
I'm not so sure. For example, the form of a triangle which we'll call triangle A can be one of which other triangles say, triangle B, C, and D, conform to by sharing some sort of chracteristic or characteristics.
Supposedly the problem is that "the form" is also a triangle, and must conform to a new form of a triangle, and so forth in an infinite regress. But why can't the first form of the triangle and the "new form" be exactly the same? Why can't we say that not just B, C, and D, but also A conforms to the new form... A! It would seem to me that the form of the triangle is simply the example triangle we most want to visualize in our heads, that has certain characteristics that other different looking triangles nonetheless share. The example triangle can conform to itself; I don't think it has to conform to a different looking triangle.
True, this requires one to assume without justification (other than intuition I guess) what the "ideal form" is. But I think Plato's point is simply that we draw from some form when identifying something, even if we don't know exactly what that form is. He's not saying the ideal triangle in our head must be 5 inches tall or anything; he just makes the general point of how we draw from a form in our heads. Maybe I'm thinking about a triangle with base of 3 inches and height of 2 inches in my head that I'm comparing an image to when determining if it's a triangle; sure, it's arbitrary, but a person is free to do that if he wants
Perhaps I'm not correct on what Plato truly meant?
I don't really have a position on whether or not the forms are actually correct; but I don't think the third man argument necessarily refutes it.