The doctors said they will try their best to help her.

try 是动词，意思是"尝试、努力"。 their 是形容词性物主代词，指代"他们的"。 best 是形容词 good 的最高级形式，意思是"最好的"。 try their best 是一个固定短语，意思是"尽他们最大的努力"。