Question Can i use two 4gb Ram stick and one 8gb Ram stick at the same time?if possible .then what should i do

CAN you? Certainly.

Will they "play nice together"? Only one way to find out, and that's to try it. There are no guarantees when you start mixing RAM, or even when using supposedly identical memory modules that didn't come together in a kit. Sometimes it works, sometimes it doesn't. The only way memory is guaranteed is in the the matched kits they are sold in. If you buy a single stick that meets the compatibility requirements, then it should work. If you buy a kit of matched sticks, then it should work.

When you start mixing kits or sticks, it might work, it might not. Nobody can tell you for sure if any mixture of memory kits or sticks will work with any certainty.

If the speed and timings of the memory are within JEDEC specs and not technically an "OC" requiring manual or XMP profile settings to run at the advertised speed, then the chances are much higher that mixed sticks will work together. Since those are DDR4 2400mhz, within the default specification for 7th gen platforms, it's likely they will, but again, there are no guarantees.