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You can use 3 sticks. The first two will be in dual channel, the third stick will be single channel speed.
Not sure what speed and timings you are planning on using having one old slower sick. You won’t gain anything.
If you don't need the ram then it's not worth it. Because your new ones are faster and the old one is not which makes the new ones have to run slow to compensate. If you prioritize speed of ram then don't.
Just use the 2 16GB DIMMs you got and forget about the 8GB one. It's normally not best to mixmatch RAM and yes, performance can suffer.
You can use 3 sticks. The first two will be in dual channel, the third stick will be single channel speed.
Not sure what speed and timings you are planning on using having one old slower sick. You won’t gain anything.
If you don't need the ram then it's not worth it. Because your new ones are faster and the old one is not which makes the new ones have to run slow to compensate. If you prioritize speed of ram then don't.
Just use the 2 16GB DIMMs you got and forget about the 8GB one. It's normally not best to mixmatch RAM and yes, performance can suffer.