There are several differences, but in short, here are a few:
Life Expectance: Architectural shingles are almost twice as heavy and thick as a 3-tab shingle with a life expectancy of 50 years compared to the 25 years of a 3-tab shingle.
Durability: Architectural shingles are more durable. 3-tab shingles are more susceptible to damage with its single layer of material, lighter weight and a wind rating of 60-mph compared to the 130-mph rating of architectural shingles.
Appearance: Architectural shingles have an additional layer attributing to its added weight and durability. This allows for the shingle to have a dimensional look to it and much better curb appeal. A 3-tab shingle is flat with a single layer of material that was cut to create the tabs. Each shingle is about 3′ wide and has 3 shingle tabs on each full shingle.
Cost: Despite the architectural shingle being almost double the life and quality of a 3-tab shingle, the cost difference is only about 10% more.