Why did Rivka say "I will go!" and not just 'Yes'?
Rashi: She meant that she would go off her own bat, whether they liked it or not. 1
It is not clear however, why this was not a contravention of 'Kibud Eim' (See Tosfos [Hadar Zekeinim]), particularly if she was only three, as Rashi learns?
רש"י: אלך- מעצמי, ואף אם אינכם רוצים: מנין לפרש כך?
גור אריה: אם לא כן, היה לה לומר 'הן'.