Well according to Verendus there are 2 more unannounced DQ for Playstaiton series, with probably one more to be announced in a future. WIth him denying any of those to be DQH3, I don't think all of those 3 games will be spin-off.
That being said I don't think SE will "abandon" 3DS install base either. Why do people think if DQXI gets released platform(s) X then it won't be coming to another one?
1. Dragon Quest X: The one for the future its been rumored to come to Playstation, there might be some exclusivity length they have to wait out for X.
2. Dragon Quest V HD: One of the two announcements, an HD remaster that is based on the PS2 Version.
3. Dragon Quest XI: Last one