This is incorrect - you want ca-cert.pem to include a list of CA certificates, like ISRG Root X1 and ISRG Root X2 from Chain of Trust - Let's Encrypt.
In a typical setup, where you're only trying to authenticate the server, and not have the server authenticate the client, you don't need client-crt.pem and client-key.pem. Just the ca-cert.
The files you see under /letsencrypt/live/domain are for your server to use, not the client.
Ah, my bad. It seems likely that these are indeed for client authentication. When I saw "only trying to do server-side authentication" I had assumed these were for the server.
Perhaps if you were a bit clearer on exactly what software you need to add these three files into, we might be able to better understand the request and provide you with a more exact answer.
I'm assuming you're using an old version of Apache.
[let's see how far that is from the truth]