What you are asking about is often called "microsites"; you may want to search for that term. There are many arguments both for and against, but in your situation, I would recommend keeping them apart, since you say that these are three different businesses, and since none of the three domain names sounds like an overarching name. Now, if you had a domain name like mychafferedtransportation.co.za, it might be worth combining them and thus reap the benefit of a a higher visibility for the overall page. But don't worry too much about SEO specifics. The most important consideration is whether this fits in your overall business strategy; whether you want to keep the businesses independent from each other or appear as one combined brand.
