I think it is just suggesting that if you have 2 modules the best place for them is 1+3 or 2+3
Obviously if you have 3 modules you have to use all 3 slots.
Buying the memory you suggest will almost certainly achieve what you are after.
OnePlus 6 review
Procreate's iPhone app offers new brushes, a larger canvas and better support for the iPhone 8/X
Best multi-room speakers for iPhone & Mac owners
Streaming : comment regarder le lancement de Xiaomi en France ?