How to install a certificate on IIS?

Question

IIS

Good afternoon!
I recently purchased a DomainSSL certificate from REG.RU for the first time . A letter came from REG.RU like this:

Hello!
Welcome to the Domain Name Registrar REG.RU!
Please read the information below carefully. The letter contains data that is used to install an SSL certificate for hosting for your domain.
Your SSL certificate is provided below
-----BEGIN CERTIFICATE-----
// many letters 1
-----END CERTIFICATE-----
Root certificate
-----BEGIN CERTIFICATE-----
/ / many letters 2
-----END CERTIFICATE-----
Intermediate certificate
-----BEGIN CERTIFICATE-----
// many letters 3
-----END CERTIFICATE-----

However, neither REG.RU technical support nor REG.RU FAQ provide information on how to install this certificate.
I tried to google the solution, but nothing helps: I saved
each certificate in a separate file from "-----BEGIN CERTIFICATE-----" to "-----END CERTIFICATE-----" inclusive and used the instruction on https://ru.godaddy.com/help/ustanovka-ssl-sertifik... However, it is not clear what to do with the root certificate. If I install only intermediate and SSL certificates (according to the instructions above), then the certificate from the IIS manager disappears after updating the list of certificates. This gives the impression that the intermediate certificate was not imported at all.
I found similar instructions, but everything ends in the same way (it is not possible to select a certificate for https).
Poke, please, with your finger, what am I doing wrong and where to dig? I don't even know what to look for.

Answer 1

Everything turned out to be very prosaic: the data from the letter (specifically, the SSL certificate) had to be converted from PEM to PFX certificate. For example, via https://ssl4less.ru/ssl-tools/convert-certificate.html. The resulting PFX certificate can be imported into IIS.
PS: The private key was created when creating the certificate request

Answer 2

There are no answers, and I'm not an expert on such issues, but just once a year, installing an SSL certificate on my site, I can say that the sent certificate must be installed using the Complete Certificate Request item in the Server Certificates section. certificates). True, why you have 3 certificates is not clear. Try each one in turn.
After installation, you can test: https://www.ssllabs.com/ssltest/index.html