You need to build a certificate bundle (certificate chain). To do so, you just have to concatenate your three certificates. Make sure to have the Root CA on top of your chain, instead it won't work.
cat rootcertificate.txt intermediateCertificate.txt \
serverCertificate.txt > fullchain.txt
If your certificate are in DER format and you want to convert them in PEM before building your chain. You can use the ssl command below to convert your certificate in PEM.
openssl x509 -in serverCertificate.txt -out serverCertificate.pem \
-inform DER -outform PEM
You can use the same conversion to convert your private key in PEM also. Make sure to change in the "inform" in your certificate format they are not DER. Default value is PEM format.