Crab-apples are good for pollination IF they overlap flowering. Not so much otherwise. If you have an early flowering variety and a late flowering crab-apple forget it. You don't need them as a third apple may work perfectly.
In any case they are beautiful trees and excellent for wildlife. You will not be disappointed with the excellent spring and fall displays, but if the space is limited I would go for a different tree or a third real apple choosing carefully the rootstock. If you want a crab-apple put it at the background. (Optical effect. It makes the garden appear bigger). If you like to watch birds while sipping your coffee in the front.
Crab-apples would be the kings of garden if they didn't suffered from so many diseases.
Here's some more reading:
https://www.cumminsnursery.com/learn-trees/fruit-tree-bloom-...
https://www.cumminsnursery.com/learn-trees/fruit-tree-pollin...