Coordinated cyber attacks, limited nuclear strikes on Washington D.C., Houston, maybe a few strategic targets. Infiltrators pour up from Mexico. China uses the citizens already planted here. Russia comes in over the top on Alaska and hits the Gulf Coast from Cuba and Venezuela.

China uses Hawaii as a springboard to push onto the West Coast. (Huh-why-v-ee for you Howlies that don't descend for our people who were there at the time of the King.)

A half million Chinese troops make it as far to the east as Utah, get stuck in the Rockies. Texas would be cut in half (north and south). Most of the US conventional forces would cluster in the Midwest and Great Plains because neither Russia nor China can afford to destroy the biggest swathes of US farmland.

From there, it'd be slugfest for two or three years. Conventional forces, guerrilla warfare. It wouldn't be pretty. Plus the US would vaporize all of Russia and most of China.