I looked in the VMware Compatibility Guide and I found several Cisco UCS C M3S systems. For example, UCS - C22 M3S, UCS - C220 M3S Rack Server, UCS - C24 M3S, UCS - C240 M3S Rack Server. All of them are supported with ESXi releases up to 6.7. So, you can choose which ESXi version you want to upgrade to. Because ESXi 5.5 general support is almost ended, I would recommend you to upgrade to ESX 6.0 or a higher version. Take into consideration that end of general support for ESXi 6.0 will be at 2020/03/12. So, if you upgrade to ESXi 6.0, you will have to do another upgrade within one and a half year.
When you upgrade to ESXi 6.0 or higher, there are several decisions you have to make. For example, do you want to use the Windows-based vCenter Server or do you move to the vCenter Server Appliance (VCSA)? Do you use an internal or an external Platform Services Controller (PSC)? Read the release notes and the documentation before you upgrade and make your decision on the information you get. Besides, upgrade the firmware of the servers before you do an ESXi upgrade.