It is actually better to keep templates from the same framework only and remove the rest.
They are both different in many aspects, such as module position, layout, so a conflict might occur.
Hence, both Helix ultimate and helix3 framework based templates should not be installed at the same time together.
You can of course use multiple template in the same site, but from the same framework only.
Also, please note that there should not be left the system plugin of the other framework while removing it. Sometimes, after removing the templates from different framework, users may forget about the system plugin which can cause a possible issue.
And, as per my suggestion, as Helix ultimate is the latest framework, you can use it. It is much more flexible than Helix3 and you will enjoy it.