Could be two separate fathers. Female cats release eggs after mating, so it is quite feasible that two or more Toms could mate with the female who would produce different coloured kittens.
We have three cats, from the same mother, my wife saw them being born.
One is a big long haired tabby male,
No.2 is a long haired black female
No3 is a short haired white and black female.
Could be, but the main issue here is the red gene. With the second litter, if there are any males in there, you can be almost certain that the little tortie ISN'T related to them.
Your three is simple as non-agouti and long hair the two most common recessive genes for cats and an extremely common combo in non-pedigree litters. White spotting is totally independent of the genes for agouti and long hair. I don't know what colour mum was, but here's a rough explanation.
The two parents of your lot... one would have been tabby carrying non-agouti (tabby being dominant over solid, so you only need one copy), and the other would have been either black or tabby carrying non-agouti. Both parents would have carried the gene for longhair but been shorthaired in appearance. Also, both may have had white spotting to some degree. If a parent has only one copy of a recessive gene as is the case here, then there's only a 50% chance of it being passed on to the offspring.
You have to have two copies of the gene for non-agouti and two copies of the gene for longhair to be longhair - ergo the black longhaired kitten. The longhaired tabby kitten only has one copy of the agouti gene, but because the gene for agouti is dominant, it manifests as tabby, and it got two copies of the gene for longhair, one for each parent. The shorthaired black & white kitten didn't get a gene of longhair, but got two copies of non-agouti and a gene for white spotting, which gives you a tuxedo cat.
The default colour / coat combo for a cat is tabby shorthaired, and it are the various recessive genes that change that. If you want, I've got some very good colour charts I can scan and post. When you see it in visual format, it's really very simple to work out.