Basically this construction leaves out some (obvious) part in the name of brevity. This is a common institute in the English as well as the German language alike:
Can you give me an example? I can give you one.
Similar your own example. Notice that the numeral takes over the declination suffix from the noun - here Dativ Plural, even though it is not capitalised.
Die Angebote findest du in allen drei[ Geschäft]en.
This works also with nouns in other Kasui:
Den Artikel haben viele Geschäfte. Drei haben das Angebot. (Nominativ)
Das Angebot aller Geschäfte ist gut. Das Angebot dreier davon ist sogar noch besser. (Genitiv)
Das Angebot findest du in allen dreien. (Dativ)
Alle hat man gelobt. Befördert hat man drei. (Akkusativ)
In case of the Akkusativ "drei" is the - shortened but modern - version of "dreie". This is the same as above: the numeral will take over the suffix of the noun.
This sort of (perhaps "elliptical" or "brachylogical") construction is possible with small numbers: single-digit and probably two-digit numbers are OK, you can't do it with, say, a million (at least it would sound very weird).