Intel's specs for that processor say that it supports DDR4-2666. They don't list DDR4-3200 as supported. Even if you could use DDR4-3200, the Crucial sticks have higher CAS Latency than that of the DDR4-2666, so we need to calculate the true Ram Latency to compare them. The Crucial DDR4-3200 has a CAS Latency (CL) of 22. The calculated Ram Latency is 13.75 nanoseconds.The Crucial DDR4-2666 has a CAS Latency (CL) of 19. The calculated Ram Latency is 14.25 nanoseconds. That's not a big difference. For tasks that use multiple cores of your CPU, there may be a slight difference in performance. For GPU rendering, it may not make much difference.  So, if you could use the DDR4-3200, for most tasks you wouldn't see a noticeable improvement over DDR4-2666. And, Intel says your processor supports only up to DDR4-2666, so if you installed DDR4-3200 it would underclock to 2666 anyway. Go with DDR4-2666.   
Hello. I am thinking about upgrading my RAM to 16GB and I want to buy the highest MHz RAM that my laptop can support. But I don't know how much MHz my laptop can support. This website shows the supported configurations and all of them are 2666MHz. However, when I look at Crucial's website, It says my laptop is compatible with 3200MHz RAMs. Is there anyone who can help me deal with this issue?
Dell G3 15 3590 RAM Upgrade