Compatibility of Shimano Ultegra and Sora components My road bike has an Ultegra groupset, but some parts need replacement (chainrings, chain, and cassette). The chain is worn out, the cassette is sliding with a new chain, and the chainset is also worn out. Can I keep my 3-9 derailleurs and mount new Ultegra chainrings with a Sora cassette (9) and a Sora chain to save some money, or should I keep riding and mount ultegra chainrings, chain, and cassette when they are fully worn out?
Most groupset parts don't wear out on a predictable replacement schedule. Derailleurs, brakes, cranks, and brifters should last for a long time, barring unusual wear or crashes. And I hate to say it, but if you do get to the point where you want to replace those parts, it might be cheaper to buy a new bike.
Modern Sora and Ultegra chainrings are 4-bolt, and are completely incompatible with your 5-bolt Ultegra cranks. You could replace the entire chainset (aka crankset), but a modern Sora chainset will be incompatible with your bottom bracket (your Ultegra chainset uses a separate BB; the Sora and other higher-end Shimano chainsets today integrate the spindle into the left crankarm), so that would need to be replaced too. You might be better off looking for NOS Ultegra rings.
Sora-grade chains are lower quality (less precise spacing between rivets), which will wear out the cassette and chainrings more quickly.
The Sora cassette will work.