My Female cat and dog keep trying to 'mate' is this normal??

[Victoria]

i have a female cat and have bought a female Jack Russell pup who is 12 weeks old. when we first got the dog the cat was in season and she kept throwing herself at the dog so to speak and expecting it to pleasure it. now the dog thinks that it is ok to attack my cat all the time (even when it is not ok) we also have a 3 legged cat who is petrified from it as it attempts to come on to him and a pregnant female cat who is in her last week of pregnancy. i don't know what to do and i don't want it to hurt the kittens.
My other cat is in season again and it is actually disgusting that a wee pup knows what to do to it :-(

What should i do??