While you may save money with the 3A, it's not a lot of money as such, but you would be causing yourself a lot of grief down the road.
Don't get me wrong - the 3A would probably do the job, but you'd be constantly on the back foot, trying to find solutions for issues that nobody else can help with because they don't have the same layout as you.
There may be things that happen on your system that don't happen on others because you run out of RAM, or because your system is just a fraction too slow and that hasn't been tested in development.
Whenever something like that happens, what tends to happen is people try help by trying out, on their system, some of the things that they think are causing you your problem. That works when you have ten thousand others with the same hardware, but it tends to die when only a few have similar hardware - just finding some of them online may become an issue.
The other problem is that as you grow into your hobby, you tend to want to do more and more complex things. You don't want your imagination curbed by your hardware. As your prints get longer, can you add a plugin to monitor the print from work? For the 3B+ there are thousands of people that can answer that immediately. For the 3A, how many could answer that? I know I couldn't