You are allocating an array of floats. But a tuple of floats is not  
an array of floats. In a tuple of floats, floats are boxed. Floats  
are unboxed only in records with float fields only or in arrays of  
floats.

So it should your code should be something like :

CAMLlocal1(t);
t = alloc_tuple (3);
Store_field (t, 0, copy_double(obj[0]));
Store_field (t, 1, copy_double(obj[1]));
Store_field (t, 2, copy_double(obj[2]));

Daniel