Hallmark is the essence or sign of something. For example you might say “Being eccentric is his hallmark”. Hallmark was also an official stamp on gold or silver to clarify its purity. Hallmark is also a brand, which is where you might have heard it. Trademark is an intellectual property that is owned by someone or a company. For example Coca-Cola is registered trademark. Benchmark is a standard or a realistic ideal. For example you might say “our company performs well against the industry benchmark”.

Mostly when we what to point to an essence or mark of something.