Assuming this is for a non-X3D AM5 or Intel CPUs, you should replace the 2x8GB sticks entirely with 2x16GB sticks. 1x16GB is slower than 2x8GB for somewhat complicated reasons to do with DDR5 multi channel memory configurations, and as soon as you go to more than 2 sticks you're unlikely to be able to run your RAM with EXPO enabled so you can loose a significant amount of performance from that. Plus, if you're using a three stick setup, the performance loss you get from running a single stick will also impact a 3 stick setup, so you'll get a double whammy of no EXPO and slower overall RAM access. If you use 4 sticks, you just loose EXPO, so 4x8GB is likely better than 2x8GB+1x16GB. But 2x16GB is always going to be better. We're talking 20\~50% performance losses from the worst case 3 stick setup, and around 10% from 4 stick setup.

If you're using an X3D CPU, the performance impact from any of this is negligible and do whatever is cheapest. Worse case you'll loose maybe 2% performance.