At those ages mine didn’t have access to mobile phones at all, and only very limited from 10/11 on a shared family Xbox. No YouTube or anything else allowed either.
They are now 14 & 12 and still don’t have any YouTube or social media and have a 1 hour time limit on their phones which covers messaging & games. Once the hour is up, there is no more.
If they want to play the Xbox they have to ask, and depending on what else they have done that day or what we are going to do, then I may allow them an hour (they know to set a timer and it goes off straight away at the end of the hour - any arguments would mean they don’t get to play next time). They also ask to watch tele, but I am more relaxed about that, particularly as they hardly ever want to watch.
We do a lot of trips out (walks, national trust, beach etc) as a family and with their friends (+families sometimes). They also have numerous hobbies - mostly sports training which keeps them very busy as they train at least once a day and quite often twice with two different sports. They also like doing stuff outside - we live on a small farm and they help with the animals, or do whittling, set up bows and arrows in the garden, have nerf battles, kick a football or play basketball. Indoors they ply a couple of instruments between them, so that’s always a good boredom cure, and one likes building Lego sets while the other is keen on drawing. Both like reading too.
I think the key is to set out the rules and parameters for screen time and be consistent with it. Also lead by example - if I don’t want my kids on screens then I also shouldn’t be scrolling Facebook or doing online shopping too much, and not at all during the day. In your circumstance I would remove all screens, go cold turkey for a while, then reintroduce tv / games console very gradually and in a limited way. Then hold the line! The younger two shouldn’t have phones and you are in control of that. The oldest, well maybe just a phone to text / call friends on - no social media, no games.