If this is a Mac Pro "trash can" from Late 2013, a Mac mini from Late 2014, or an iMac from Late 2015, you can upgrade it to Monterey. Monterey is no longer one of the "most recent three", but still is recent enough to be quite a bit more useful than El Capitan.
Worst case is that you have a much older Mac for which El Capitan is the end of the line – and it is past time to replace that Mac (or set it aside for running old 32-bit software like games).
If you go into the About This Mac dialog in the Apple () menu, what does it say about your Mac's model, and hardware release year / time of year? That will determine how far you can update – if at all.