As a stepmother of 2 and a mother of 1, I get where you're coming from. It's always hard for kids to bounce from one house to the next, with different sets of rules. Some kids also are very clever at using that to their advantage, telling parent A that a specific activity is always allowed by parent B. Some kids may use their electronics (or any other toy/item) as a transition aid, that is, almost a security symbol that they can hang on to in the shift from one environment to the next. With electronics, kids also have the opportunity for escape into the device or the virtual world so that they don't have to really deal with the new home. (They may do the same thing going back to Mom & Stepdad.)
I completely understand the frustrations of being a stepparent. And I'm going to say this with compassion and understanding: you have to back off. The common denominator between the step kids and your kid is...your husband. He is the only one who can take their stuff away, tell them that this behavior or that are not permitted, to not get manipulated by their freaking out (which is the teen version of a tantrum). Yes, they are hormonal, but so what? The 16 year old is going to be getting a driver's license soon - are you telling me that tantrums and use of electronics and the inability to calm the hell down are going to be permitted while behind the wheel? He's your best bet, actually - because he can see the signs of maturity on the horizon.
Your husband has to put down family rules for all 3 kids. Period. As much as it kills you, don't speak. Don't weigh in. Don't let your husband say, "Your stepmother and I think that_____." You say that to your son, absolutely. But your husband handles his kids 100%. Otherwise, you will be the bad guy. He has to find his backbone and make rules.
Now, I know damn well that the teens may say they don't want to come on the weekends or even be there mid-week. I know that's a risk. But you have to hang tough. It will get worse before it gets better. But at some point, those older boys need their father, they want car privileges, they want rides to the mall or new fancy sneakers or something else. Make them earn it. They are NOT going to be allowed certain things in your house, period. You can set a certain number of hours to use screens (iPads, computers, whatever) but you must set aside some times (maybe dinner until the dishes are cleared and put in the dishwasher, maybe a ban 90 minutes before bedtime because of sleep disturbances, maybe some tie-in to school performance or chores). It's a house rule. Period. That means you and Dad and the littlest boy aren't on those devices either - the one boy has a point in saying that you don't have to adhere to the rule so why should he. He's approaching adulthood and he's savvy. I'm not saying you never use a computer, but that you can model the behavior you wish to see. "I pay the bill" isn't going to cut it on a daily basis. They can sulk at the table if they want to, but there's no option. The more they protest, the more they whine or disrespect, the longer the time is. You have to learn how to handle his disrespect without caving in. You have all the power - you just need to use it more quietly and calmly, with more patience. Wait for the infraction and the consequence - you don't always have to hit him over the head with it by nagging or yelling. Calmer, but resolute.
I'm having some trouble with some of your words/typos. I don't know, for example, what hbd,e means. But I think you need to look at the word "cave" and why you are considering it - and why you are letting these kids run your life. Without disrespecting you and your role with the younger child and as your husband's partner in equality, your husband must handle these boys, not because he's a man, but because they are not (sorry!) yours. Doesn't mean you don't love them and care of them and have something to contribute. But all of that has to happen behind closed doors. You can remove your child form scenes he shouldn't witness, and you can ask/insist that your husband take the older boys out of the room (perhaps the room where the snacks and TV are!) when they are out of control or inappropriate. But you all, as a family, need screen rules and electronic black-out times.
The longer you wait, the less influence you have. Don't wait.