This is a good question ?
Cus universal car seats are NOT compatible with all cars.
You are best off going to Mothercare or Halfords and they(if doing their job correctly give a demonstration of how to fit the seats.
They will also know which seats are compatible with each car.
Oh and welcome to the forum 
Peugeot 405 and some BMW will not take them in the back, Omega will take any of the common brands, I know through experience. The important thing is to make sure they are fitted properly, use your full weight to push them into the seat then fit the seat belt, the baby seat should not move. I have used many types of seat and, for many years, in an Omega Estate. My own child and for work. Though for work there are some situations where I am exempt from using them.
Whether or not you will get 3 plus a booster along the back seat depends on the makes of the seats, some are quite basic and narrow but most are getting ‘quite posh’ and are wide. They may physically fit but you may not be able to get at the belt buckle for the booster seat. I currently use, daily a Citroen C4 Grande Picasso 7str, (long story) two baby seats in the back seat but can’t use the booster in the middle. Use one of the rear rear seats instead. Never tried in the Omega as I don’t use it for work but seat is wider so you may get away with it.
Best advice though, as has been mentioned, is to go to a reputable dealer, they should have trained staff to advise and fit.