Do you mean arcmap 10.3 or hec-georas? I tried to install hec-georas 10.3 but I guess it's hardcoded because the install wizard will just shut down after it finds out that I'm running arcMap 10.4.
I'm running the trial version of arcMap so it only allows me to download the newest version, 10.4. However I think I found a way around it though since my university apparently gives out temporary licenses for arcMap 10.2 so I guess downgrading is the easiest way around it.
thanks for the reply!