When used with a number, "homeless" needs to have a noun to modify.
a': I spotted three homeless men in this area this morning.
b': Three homeless people were arrested for trespassing on private property.
However, "homeless" can often be nominalized when preceded by the definite article.
c: She's been working as an advocate for the homeless for thirty years.
d: Because of their sheer numbers, the homeless have become a force to be reckoned with in our local politics.