i just upgrade my cpu finally from 7700k to a 5700x3d  my ram setup is  2x8gb 1x16gb all at 2400mhz

I guess is too low speed for a new procesor where can handle 3200 and 3600, is really worth the upgrade in performance?

also probably in case of an upgrade is better to get 2 x16gb and have a better use of the dual channel  better than use 3 spots like now 

 

In theory yes, but I have the 5800X3D and there was no measurable impact (in measured memory bandwidth and in X-Plane) from running my 3600 RAM at 3200. Others might know better but I believe that this chip does not make use of faster DDR4 RAM.

Dual channel: yes, you should try to have pairs of equal RAM in the assigned paired slots. If you are unsure just run a RAM speed test with and without that 3rd slot taken and in the correct paired slots or wrong ones. You should be able to measure a clear difference (Dual vs. Single should have ~+50% bandwidth). I run 2x32 + 2x8 btw...