The terms square and rectangular may be a bit confusing. All three of these breeds are bred for working and none could be considered "dainty". Aussies tend to give more of an appearance of stockiness though. I describe them as "square" because their height to length ratio is closer to 1:1 than BCs and ES who both tend to be longer than tall.
Both BCs and ES tend to be "leggier" than Aussies although some of that is an optical illusion because the heavy Aussie coat can obscure the legs a bit. Young dogs of all three breeds tend to be leaner so knowing the age will help determine if a dog is in that awkward teenage stage or is really that lean and lanky.