Certainly, divide into three - each domain in a separate hosting acc
If all three domains are in the same hosting account then these resources are added together so the chances of reaching the limit (HITs, CPU, RAM, etc.) are much higher, which can result for example 24-hour suspension.
Each hosting account has its IP that is assigned to you, so in case of problems if one or more IPs are down there is less chance that all three websites will be down, unlike if they were all in the same hosting acc (same IP).
In addition, it is convenient for you to move websites from one acc to another in case some IP has problems for a long time.
Security - for each hosting account you are assigned a different FTP pass and user name.
To avoid confusion - when I mention the words “hosting account”, I don’t think that you create an infinityfree account three times with different email addresses.
I mean the hosting account that you add inside the client area (you can have max 3).