Can anyone give me an idea as to what is the best way to transport a cake by car for a nearly 3 hour drive. This is going to be my first time making a wedding cake for a friend , but I need to move it from Houston to San Antonio (and we have the heat to worry about)

If you can find a cooler(s) big enough, that would solve your problem. You can put blue ice in a transport box with the cake, but you have to figure out how to fit in the ice so it won't slide and hit the cake. Under an inverted pan with the cake on top-on non-skid material can work.

put something heat reistant over the boxes-like those sun shields for windshields.

When I have to transport a cake long distance I fill 2- liter bottles with water and freeze them overnight. Before transporting, I let the air conditioning run full blast in my car, place the cakes in open top boxes and place the 2-liter bottles all around the boxes, securing them with towels. I have done this several times and have had no disasters.