I have to take a bus to work today.(good)
I have to catch a bus to work today.(good)
I have to get a bus to work today(not too good)You get on the bus but you don't easily "get a bus". But you can take or catch a bus.
To me, if I "take a bus" or "catch a bus" I mean basically the same thing - that I will be riding a bus.
I can see the possibility of "get a bus" in certain contexts - for example, let's say that I have been given the task of arranging transportation for a large group of people. If I "get a bus" that means that I will be acquiring one for their use and may or may not be riding it myself.