You haven't clearly identified which Mac model you have. Look in "About This Mac" and tell us the name which appears in bold on the line after the OS version number. It should start with the text "MacBook Pro" and include a model year in the detailed part in the parentheses.
Looking through a database of Mac models, I spotted two MacBook Pro generations which include a model with a 3.1 GHz Core i5:
- MacBook Pro (13-inch, 2016, Four Thunderbolt 3 Ports)
- MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports)
Both have a dual-core processor, which is not a good fit for running virtual machines (you should be reserving one core for the host, leaving only one for the virtual machine, and many OSes need two cores to perform well).
Both models are supported by all later macOS versions to date (Mojave 10.14, Catalina 10.15 and Big Sur 11). Big Sur's raised the minimum to 2013 to 2014 Macs (varies depending on the family), so a 2016 Mac will hopefully have at least another two years of support in future macOS versions.
There is no Fusion 11.3. Do you mean 11.5.3, or something else?
Fusion 11.5.3 is the last version which runs on High Sierra. Fusion 11.5.5 to 11.5.7 require Mojave or Catalina. Fusion 12.x requires Catalina or Big Sur.
Fusion 11.5.5 to 11.5.7 are free updates to earlier 11.x versions, so if you were to upgrade to Mojave or Catalina you could just update to Fusion 11.5.7, but 11.5.3 would still work if you didn't want to update for some reason.
Big Sur requires Fusion 12. If you upgrade to Big Sur, Fusion 11.x will not work and you won't be able to use any of your VMs until you upgrade to Fusion 12. You can't install or run Fusion 12 on High Sierra (or Mojave).
There are some fundamental differences between Catalina and Big Sur for how Fusion 12 works: in Catalina, VMware's own hypervisor is used; in Big Sur, Fusion uses Apple's hypervisor. This is causing a fair amount of teething trouble for many people running Fusion on Big Sur in areas such as networking and nested virtualisation. It isn't clear how much of this VMware can fix, or whether we need to wait for Apple to fix or improve some parts of their hypervisor.
I'm waiting out the these issues by not upgrading to Big Sur on any Mac which needs to run Fusion.
Upgrading to Catalina would allow you to keep running Fusion 11.5.3 (or free update to 11.5.7) temporarily until you are ready to upgrade to Fusion 12.
There is a possible catch: if you have older 32-bit applications running on your Mac (not inside a VM), they won't work any more if you upgrade to Catalina or later, because Catalina dropped support for 32-bit applications. (In some cases you can keep them working by running an older macOS version in a VM, but some applications cannot work this way because they depend on features not available in a macOS VM, such as 3D graphics hardware support.)
