I think the OP is asking if he/she will be able to finish residency in 3 years, being board certified in ER, if the OP starts out by doing a TY (either ACGME or an AOA one). I would think the answer is no...although I think the OP should check with the American Board of Emergency Med (isn't that the accreditation body for ER docs?). I don't know about the AOA vs. ACGME TY thing...although I'd think it will be hard to a get an ACGME TY in the scramble...usually those fill up with competitive candidates, like people doing rads or derm. If there are any spots left open, would think they would be at a less desirable program (i.e. one that doesn't have many electives, etc.).
There probably are some ACGME internal medicine prelim spots that are still open, that might be decent and that the OP could likely get...a prelim medicine year wouldn't be great, but I'm sure the OP could get SOME credit for it later if the OP switches to ER. Most medicine prelim years would include at least one MICU month, and often an ER month, too. Of course some of the IM ward months would count for the OP as well. The advantage of doing an IM prelim year vs. a TY would be that the OP is 1/3 the way through an IM residency, if things fall through and the OP isn't ever able to get into ER (sorry to be harsh, but it's worthwhile to at least consider the possibility).