Which one is right "Comment" or "Comments"

Solution 1:

Certainly the first option is correct since "a" implies one -- one comment. The third option is also correct. It is just more encouraging to provide more than a single comment. The second is the most awkward to me. It seems slangy. I would have expected it to either say "Submit a comment" or "Submit comments". Contextually, I think they are all basically correct. The singular phrase treats the whole submission as a "comment". The plural phase acknowledges that there may be multiple observations in the submission. Use what fits with the rest of your site.