If you set the sticks in the right DIMM slots, then the 16Gb will run in dual channel. Anything that's used over that amount will then bump into the 8Gb in single channel and you'll take a small performance hit. Chances of that happening with gaming is extremely slim, I can't think of any game that ram intensive.
But as velocityg4 stated, that's if they play nice. Mixing kits is always a crap shoot. Most times they work, sometimes they need adjusting, sometimes they don't play together at all.
A kit is a single blister pack containing 1/2/4/8 sticks that have been factory tested. If you buy 2x blister packs, that's 2 different kits, doesn't matter if they are exactly the same brand, speed, timings, model, size, they are different and not tested by the factory as working sets.
With the 16Gb at 3000 and the 8Gb at 2400, if you put all 3 sticks in, if they play nice then all 3 will run at the slowest speed and timings.
At this point, you'll get better performance by not including the 8Gb.