i take it that you are talking about the dds/md program for oral and maxillofacial surgery. it's the only dds/md recognized by the ADA. first, enter and complete dental school (it could be a dmd or dds). then, you have to apply and match into an OMFS specialty residency that specifically offers that degree. the dual degree dds/md residencies usually last 6 years past dental school. OMFS residencies that do not offer the MD are more numerous and only last 4 years post graduation. as for the program itself, you start by being dumped into the second year of med school. you then finish med school, pass your boards, and do both surgical and OMFS residencies. sounds like fun, huh? if there is an upside, whereas most 4 year OMFS programs charge tuition, the 6 year dual degree programs usually pay residents on the PGY scale for the duration of the dual degree residency. that amounts to between $35 and $50K a year, with no tuition. in my research, most dual degree programs only accept one resident per year.
i worked for an OMFS with a dual degree. he made around 500K per year. but when asked, he would not do it all over again if given the choice. he went through ALL of dental school, 3 years of med school, a surgical residency, and the OMFS residency, just to work from the neck up. he didnt get done with school until he was nearly 35. in retrospect, he would have been more satisfied had he just been a general dentist, or just a general physician. but not both. of course, if sure that not all of the OMFS's would agree.
4 or 6 year program is personal choice, but even at 4 years it's still longer than all of the other ADA recognized specialties.
as for dental school after 3 years and without a bachelors, many schools still allow this. however, they expect non bachelors applicants to be of a very high caliber. some schools would require you to finish the bachelors degree while in dental school. if you have good stats and you dont mind spending the money to apply, give it a shot. generally, the standpoint will vary from school to school.