Is 3600mhz memory stable for R7 3700x ?

Depends on the sticks of RAM, 3600 is a sweet spot, about 1.35v

teots said:

And what is the default voltage of 3200mhz memory ?

Your Motherboards support list of QVL ram tells you the default voltage & speeds.

teots said:

I need a little advise here, my system sometime crash with 4 x 8gb cosair VENGEANCE LPX 3600mhz c18 ram with xmp.
But no problem with 2 x 8gb. What can i do about it ?

Did you buy all four sticks as a set? Two sets of 2 x 8GB sticks, usually isn't as stable as one set of 4 x 8GB sticks.

teots said:

I need a little advise here, my system sometime crash with 4 x 8gb cosair VENGEANCE LPX 3600mhz c18 ram with xmp.
But no problem with 2 x 8gb. What can i do about it ?

How long you had the ram ? let it run in for a few weeks first that may help ?

I had that problem with 4 sticks so I upped the speed without using xmp (if it lets you ?)

stormy13 said:

Probably find the IMC doesn't like that speed with all four slots populated. Try running with the four sticks but only at 3200 MHz, and see if it is stable there.

Yep, looks like you must stay away from 4 slots, unless you have a Threadripper, X79, X99, X299 or the like, which supports quad channel.