Thank you both. I understand the multiples of two and multiples of three part.
I just wasn't sure that this particular build wouldn't be dual-channel instead of tri channel.
Before purchase, when researching dual channel v. Tri channel, it didn't seem like the difference was all that big.... but it also seems the posts I was finding were 1-2 years older.
Is tri-channel reasonably faster than dual-channel in real world use?
So, ideally, with four existing sticks in the coming PC, I'd throw away one stick, leaving three remaining. Put them in the (for example) 1-3-5 DIMM slots.
Then, I can order, say, a 12GB/3-stick combo from Crucial, put those three sticks in the 2-4-6 slots.
That'll give me 18GB total, with two sets of three matched to each other. My understanding is that two sets don't need to match each other, as long as the three sticks within
each set do.