There are definitely routes where 3 class service has traditionally been available all the time, mostly between hubs.. between IAD & SFO/LAX, DEN-ORD/IAD among them. Typically hub to hub flights. Seasonally you sometimes can get 3 class service between SEA & IAD.

The only sure fire way to know is to check the seatmaps (unless the equipment shows as a 747..then there is a 100% chance it's 3 class).