Please read this other thread entitled "Do I need to install RAM in pairs" to get answers about populating the four memory slots, performance comparison from "single channel mode" vs. "dual channel mode" (reputed to be 10-15% faster than "single channel mode"), etc.
The two harder to access under-the-keyboard slots (#3 is the upper and #1 is the lower, with #3 lying over #1) are where Lenovo installs all 1x or 2x memory configurations that you purchase. If you buy a 1x setup (either 1x8GB or 1x16GB) that stick will be in the #3 slot. If you want to add your own second matching (size and speed) memory stick you should do it in the other "paired" slot of this pair, namely in #1, if you want to have optimal "dual channel mode" performance. That means you have to get to this location under the keyboard, which you already know is more difficult.
Lenovo 1x8GB=8GB and 1x16GB=16GB configurations involve only one memory stick, delivered under the keyboard in #3. You have to go up to at least 32GB from Lenovo (2x16GB=32GB) in order to have the factory populate both #3 and #1 under the keyboard for you. If you go up to 64GB (4x16GB=64GB) all four memory sockets will arrive populated. Anything else is do-it-yourself, and you now have the information you requested about sockets and their usage.
If you just go ahead and order some more expensive 2x memory configuration from Lenovo (i.e. 2x16GB=32GB or 4x16GB=64GB) then Lenovo will at the very least put those two memory sticks in this under-the-keyboard location (#3 and #1) or will populate all four slots (i.e. #3 and #1 under-the-keyboard as well as the two easy-to-access #4 and #2 sockets directly accessible on the underside of the laptop). Note that #2 and #4 are not only easy to get to simply by removing the cover on the underside of the laptop, but they're both "flat, side-by-side" so you can easily insert two memory sticks into these two sockets.
Dual-channel mode performance requires a matched pair of memory sticks in either or both of the "pairs" (i.e. using just #3/#1, or just #4/#2, or all four slots #3/#1 and #4/#2). You can have two different sized memory sticks in each "pair" (e.g. one pair can be 2x8GB and the other pair can be 2x16GB, but each pair should have two identical cards), but each pair of sticks must be the same size and all of the memory sticks must have the same speed (else the BIOS will simply run the faster memory at the lesser slower speed of the slowest memory). Any other odd configuration (e.g. 1 or 3 sticks of memory, mismatched memory stick sizes or speeds, etc.) will result in the slower single-channel performance.
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