Yes, using three RAM sticks is perfectly fine.
Yes, using three RAM sticks is perfectly fine.
I've got your setup and the upgrade plans in mind. With your current configuration, you're using two 4GB modules at 1333 MHz. Adding an 8GB module would bring your total to 16GB, leaving a spare slot for future features. Using three sticks is fine as long as the system supports it and you have enough power delivery. Just ensure your motherboard can handle the increased capacity and voltage requirements.
You have the option but should avoid it. You require: either two modules arranged in odd/even setup for dual channel, identical speed and production date, or four modules with matching speed, brand, and production date. Adding a third module reduces it to single channel. Also, mixing single ranked and dual ranked modules isn't advisable. (Chips on one or both sides) Purchase in pairs when feasible, since the performance gap is substantial enough to observe.
Your RAM functions properly in flex mode. The two 4GB sticks operate in dual-channel, and the single stick uses single-channel, which may cause slight delays when accessing data from the larger 8GB stick. Make sure you install the stick in the correct slots on your motherboard.
It doesn't always drop in just one channel. Occasionally it functions in a "hybrid single/dualchannel" setup... the total on both channels adds up for dual... for example, 2 x 4 GB on the first, 1 x 4 GB on the second—this gives you 8 GB in dual, with the remaining 4 GB slower in single.