If I were Rockstar and want to keep Cash Card sales on the top, I wouldn't give you 3rd garage.
So you'll be forced to sell your cars in order to keep other cars you wanna try.
Remember, when you're selling your vehicle, the amount of money you get back will never cover up the amount you spent for the vehicle purchase (if purchased) + mods
The only time I bought a cash card was when they added the 2nd garage option. If they give us a 3rd garage. I will gladly buy another cash card to fill it. I currently have 40 cars between my 2 characters and want to add many more. I love cars. That is my favorite part about this game. If I had the space I would get all 7 of the new dlc cars. But decided to only sell 3 for my favorite 3 of the new dlc cars. After selling 3 cars it doesn't take long to grind enough money to replace them. Rockstar could be selling allot more cash cards if we could buy as many garages and cars as we want.