SSL 证书的公钥 domain.com.crt 里面的 3 个证书都有什么区别？

加速 https 需要上传 SSL 证书，打开公钥 domain.com.crt ，发现里面有 3 个证书：

-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----