长(zhǎng)辈(bèi) means ‘the elder people’ as 长(zhǎng) expresses the meaning of ‘older/elder’.
前(qián)辈(bèi) means ‘senior/the person with more experiences in the certain field’. For example,
Suīrán tā hěn niánqīng， dànshì tā shì zhège bùmén de qiánbèi
Although he young but he is this department senior
‘Although he is young, he is the senior of the department.’
先(xiān)辈(bèi) means ‘ancestor’ as ‘先(xiān)’ means ‘before/original’.