Sorry it's taken me so long to respond. Thank you for sending the links to these articles, both of which use the phrase "several homeless". I find this slightly less objectionable than "three homeless". However, I should have made it clear that I find these expressions awkward sounding, but would not actually call them grammatically incorrect.
The first article, "Care Packages for the Dallas Homeless", is well-intended but rather carelessly written. It would have benefitted greatly from some copy editing. This is a shame, as such poor writing as this can distract readers from a truly worthy cause.
Interestingly, I don't object to the statement "[t]here are over 3,000 homeless in Dallas". It seems that nominalization of a quantity of homeless becomes more acceptable the higher and less precise the number is. For example, I would never call "a homeless" acceptable. "Three homeless" is probably not grammatically incorrect, but it grates on me. I don't particularly like "several homeless", and would never write it, but I wouldn't correct someone for saying it. "Thousands of homeless" sounds fine.
Unfortunately, I can't cite any rules to support these opinions. I'm just going by what sounds natural to me. I hope that David or Gustavo can add something more definitive to this.
In the other article, "Wrestling with Seattle's Homelessness", I attribute the nominalization of "several homeless" to journalistic tradition, which places a premium on conciseness. This tradition has its roots in the days of manual typesetting, when finding a way to express something in nine characters instead of ten literally caused a ten percent savings in labor costs. (This is why newspaper writers tend not to use the Oxford comma.) The only thing I find serious fault with in this article is the title. It indicates that the city itself lacks a home, not that some of its residents do.
So, I need to back off considerably from my earlier statement:
When used with a number, "homeless" needs to have a noun to modify.
But something must have struck you as wrong with the construct, or you wouldn't have asked the question.