I guess by a2, b2 and c2 you mean a2, b2 and c2, because a2+b2=c2 (Pythagoras).
To find any of the sides a, b or c, you need to know the other two. For example, if you know a and b, c=√(a2+b2) and may not be a rational number. You can also find a if you know b and c: a=√(c2-b2). Some triangles have rational length sides, the most popular being a=3, b=4, c=5, because 32+42=52, that is: 9+16=25. There are an infinitude of triangles with such rational length sides.
If you are only given c or c2, there are many possible segment lengths for a and b because there are many numbers whose sum is c2, but most of these will be the squares of irrational numbers, that is, not integers or fractions.