There isn't that much of a difference in the three forecasts, except for the Weather Channel's. The Weather Channel, however, is based in the US where they don't have to figure headaches like windchill out. This explains why their low for Monday is a full 9 degrees off from the lows of the other two forecasting services. Other than the windchill, the three forecasts' highs and lows are within a couple of degrees difference. This would be normal as the calculations with the forecasting formulas will differ based on information on hand at the time.
Weather Forecasting began with early civilizations monitoring meteorologic and astronomical events to monitor changes in the weather based on the season. Babylonians began trying to predict short term weather around 650 B.C. based on the appearance of clouds and 'halos' (No, they weren't playing video games, they looked at circles of light surrounding an object; in this case, usually the Sun and the Moon). People tried to predict the short term weather using their experiences and folklore up until the end of the Renaissance, when it became evident that these methods were inadequate, and that to accurately predict short term weather, they needed to increase their knowledge of the weather. After that, they continued to refine their instruments and methods for forecasting the weather while observations and technology increased for the atmosphere. (Want a deeper summary? Me neither, but here it is anyways)
Now I'll discuss some weather lore. The first, probably everyone has heard: "Red sky at night, sailor's delight, Red sky at morning, sailors take warning." This saying observed that when the sunset was red, there would be dry weather the next day; and that when the sunrise was red there would be bad weather that morning. The second saying is just a myth and actually doesn't have any scientific evidence to prove it. It is: "Cats and dogs eat grass before a rain.". While some observations can of nature can sometimes predict the weather, this is not one of them. Cats and dogs normally eat grass, as they require some vegetation for their health. If you want, you can look on Wikipedia for the scientific explanations for these sayings. (I know Wikipedia isn't allowed as a school resource, but that article is actually very interesting)