@ Kev,
How do you know or what makes you think the other two are mixed breeds? I am asking so I can be better informed.
It's familiarity with the breeds, none of these match the 'usual look' for alleged breeds. Others have already mentioned the shape/fluffiness for 'lorp for example. Buffs feather out very obviously buff- light gold color.
That barred rock does come close however his barring, leg color and weight is a bit off.. if pure, then he's quite small/lightweight. But very typical for cross or mixed breed. There is also a very common problem of a specific color being assumed to be a specific breed, barred birds being one example. Almost any bird with his coloring willl be called 'barred rock' by someone even if the bird is say, a cuckoo/barred game cock or cuckoo Marans or..
Try looking up breeds you're interested in, look at pictures of day old chicks, young chicks, juveniles and adults of each breed and color within breed of your interest. To be fair, it will be easy with some- such as comparing buff orps and the bird sold to you as such- vast difference. Others won't be as easy and takes some familiarizing especially for comparing your barred rock with others.. he will be the hardest. with some comparing, the australorp difference will become apparent.
For most fool proof way to get pullets is to order them as pullets from hatchery or see if a feed store near you will have sexed chicks or find a breeder who will be honest and knowledgeable and also happen to have chicks that will be sexable at hatch, such as barred or cuckoo.
Pure barred or cuckoo lines will produce male chicks that are visibly lighter than female chicks right at hatch.
If someone is doing a careful sex linked mating such as a RIR rooster over Barred rock hens will give sexable chicks at hatch(males have a white head dot, females don't). Be aware, the breeds reversed will NOT work.. Barred rock rooster over RIR hens will give all identical looking chicks. Same goes for a mixed breed flock of several different breeds or colors... that will not give reliably sexable chicks.