[SalesForce] Issue in callout to an external REST service over HTTPS from Salesforce

I have to make callout to an external REST service from salesforce. Service provider has shared an 'HTTPS' endpoint with me.

I am able to access the service by simply pasting service endpoint in browser, and it returns a JSON response. But when I try to invoke same endpoint from Salesforce it does not work and gives below error:

sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

I contacted my service provider and they mentioned that the service is working in browser because the browser trusts my service provider's certificate and similarly Salesforce should also trust the certificate.

I downloaded the certificate from the browser and tried to add it to callout as in Approach-1 and Approach-2 below. And also tried to add a salesforce self-signed certificate as in Approach-3 below. But every time I get the same error.

HttpRequest request = new HttpRequest();
String endpoint = 'https://serviceprovider.com/querystring';
request.setEndPoint(endpoint);

///Approach-1////
//String cert1 = 'certificate details';
//request.setClientCertificate(cert1,'password');

///Approach-2////
//String cert1 = 'certificate details';
//String cert2 = EncodingUtil.base64Encode(Blob.valueOf(cert1));
//request.setClientCertificate(cert2,'password');

///Approach-3////
request.setClientCertificateName('Testing');

request.setMethod('GET');
HttpResponse response = httpProtocol.send(request);
System.debug('######'+response.getBody());

Any help to resolve the issue will be appriciated.
Thanks in advance.