If there were any previous indicators that there should be a reason to expect J-XD to have three engines then I think we could have taken the imagery and associated vague "approvals" with a bit more weight.
Such a major design choice/deviation only being signaled now this late into the cycle, is not impossible but it's odd.
Unless we have that, or if someone of credibility outright says "it will have three engines" (or something to that effect), it is worthwhile to note that any number of other features on the art (large, tailless design for example) could be seen as "sensitive" even if it's in the ballpark of "close enough". Considering how little we know of what J-XD will actually look like, and considering how the various configurations it may have are most likely to be variations of high speed flying wings, even informed guesses are probably going to get us relatively close to the money.