Cashew isn’t a seed, its closer to a nut.  Technically Its a drupe which is a type of fruit.

Plants have developed clever ways to disseminate their “offspring” and fruits nuts and seeds are vastly different in this regard.   
Oily Fruits like olives & avocados are tasty to get animals to help disperse them.  Nuts are technically fruits so same kinda method. 
Seeds on the other hand are VERY different.  They use toxins to discourage animals from foraging them.  Some Birds have adapted but mammals generally haven’t. 
Manufacturers then use detergents to neutralize and remove the bad tasting toxins (bitter) and they also boil the seeds to reduce the oil out further damaging the oil. Cold pressed flax is good for example but it’s expensive to produce in large quantities so larger companies that need a cheap filler buy things like canola oil instead because its cheap and people don don’t immediately get sick from it.  Its a double whammy though. Firstly its rancid already from heating it AND its got many toxins that cause inflammation. 

No oil is good for you after its been heated past its smoke point but the delicate fats (omega-9) in seeds are damaged from any heat. 

As a general rule cook with omega 3 and as the temperature goes down, use omega 6 and avoid eating a lot of omega 9. 

Heres an example: Smaller quantities of sunflower are okay because they’re loaded with other nutrients and minerals that are hard to het in out diet but don’t heat them just eat raw. If the seed came from a fruit like a pumpkin or sesame fruit you’re good to eat more of them just remember that heating them fucks the oil up. 

 When you say "cook with omega-3", what do you mean, cook with fish oil, flaxseed oil or soybean oil? 

 Just don’t heat any oil past its smoke point. Omega 3s are the most stable so if you need to marinade for a BBQ  or fry, choose an oil thats going to stay stable.  
A quick search will answer what oil is best for each and just avoid seed oils in general especially when cooking. 

 Animal fat is the best cooking fat when in doubt.  

 You posted a whole lot to be wrong about a cashew not being a seed. You're contradicting yourself. You're right about it being a drupe, which is a type of fruit, and a cashew is a cashew fruit seed. It is botanically a seed.