I have found that any alignments created in SS3 lose their geometry information if you open the dgn in SS2.
It appears that SS3 is not backwards compatible.
Also, if you create an alignment in SS3 using the civil geometry tools and then open the dgn in SS2, the civil geometry tools are not available then in SS2 - the geometry lines you created are just shown as smartlines.
The moral of the story is - if you start using the civil geometry tools in SS3 you will have to stick to SS3.
Therefore, be careful !!