The first thing to know is how the load is spread across your  3 backend servers. Is it based on DNS round robin, cookie sessions or IP source addresses?

It's not the client (here NeoLoad) that decides which server to hit but it's based on the above rules that may be in place on server side. Can't you check on the backend servers or with your team about the rules applied to spread the load?

So based on the rules that are applied you may have to configure NeoLoad differently. For example if it's DNS round robin you may have to override the DNS cache at the zone parent level. For more details you can check the NeoLoad documentation here

If the load is spread based on the IP addresses you can use multiple load generators or IP Spoofing mechanism in your LG as described here