We have a "Parent Company" as a holding place, and 3 subsidiaries that were separate companies under similar but different ownership. Now, one of these subsidiaries has come under ownership of another of the subsidiaries. Because of this, we need to run consolidated financial reports for 2 of the subsidiaries but not the 3rd one. Is this possible?

you have to restructure freeing the third sub out of its parent. Create another parent sub (above your current parent, only for the structuring purposes) and link your third sub directly to it.