The Allies have always been a faction of high-tech might in the Red Alert universe, and Red Alert 3 shows no deep differences in this regard. The Allies have powerful units utilizing Spectrum Technology (essentially Prism technology), Mirage Tanks, and Dolphins.
The Allied build mechanic is unlike any seen in C&C before. The Construction Yard gives a large build radius, and additional structures don't grant any additional radius, leading to compact bases (only the Prospector can increase build radius when deployed). Structures queue in the sidebar when the purchase icon is clicked, and can be placed when construction is complete, as in C&C 3. Lastly, most tech upgrades are done at the Construction Yard, but they only affect structures in the Construction Yard's radius, meaning that secondary bases either require their own Conyards or are limited in terms of what units they can produce.
Detailed Faction Information:
RA3 Faction Information: