Can I Romance Ashley even though I did not in ME1, and I did Romance Miranda in ME2? Or am I stuck with Miranda?
Yes. You can abandon your previous relationship with Miranda to pursue Ashley in Mass Effect 3.
As a rule of thumb, you can initiate a romance with characters returning from Mass Effect, who were romance options in that title, and new characters introduced in Mass Effect 3. You cannot initiate a romance with characters introduced in Mass Effect 2, or characters who were introduced as romance options in Mass Effect 2, but you can choose to continue those relationships if you began them in the prior title.