With three dogs you will obviously have to balance various factors. The law (in the UK at least) only says dogs should be kept under control in the vehicle which means tethering them so they don't disturb the driver. However you'll doubtless also be thinking about safety, both of the dogs and yourselves.
You have broadly speaking three options then: crates (either in the rear or in the 'living area'), harnesses, (in rear or on bench seat), or just light tethering.
If you choose harnesses, be under no illusions that unless they are of the right specification they will NOT protect the dog or you in anything other than a trivial shunt. I researched this a couple of years back. Most of the harnesses sold, even "good" brands, are not crash-certified and when independently tested have been show that they are utterly ineffectual, they WILL fail instantly and your dog will become a projectile (a likely scenario is your 10kg small dog hitting the back of your seat, or your head restraint, with a force similar to that of a third of a tonne weight dropped from about two metres). There are now a few harnesses that have actually been designed and tested to be effective. So choose carefully.
Similarly with crates. Even if the crate is secured effectively (hard to do), the dog will be thrown forward in the crate, the crate structure will fail and if the front of the crate isn't against a hard object (eg the beach seat back) the whole dog/crate unit will become the projectile. Your driver/passenger seat will give you some protection against that - assuming not a rollover accident - but be under no illusions that a crate itself is likely to protect the dog in an accident in any significant way.
We only have one dog so it's much easier. Mostly she travels on the bench seat in an EZYDog harness (uses the seat belt as the anchor). But on a long journey we sometimes let her sit/lie on the floor behind the driver's seat (just lightly tethered) as she often prefers that and we're prepared to accept the risk on less risky roads like motorways.
There will be some forumistas who are in the multi-dog scenario like you, who can tell you what they do.
Sorry if what I've written sounds scary but it's as well to understand the real issues and then make choices accordingly. One approach being just to accept some risks for convenience, comfort and holiday fun. Your dogs will love Cali-camping!