In all my years in Thailand I've literally never heard or seen ANYONE put KB for Kasikorn Bank. How am I supposed to know it's Kasikorn Bank? Why not Krungthai Bank? Why not Krungsri Bank? Maybe there is some kind of account or investment firm with the name or initials of KB. So No, I am not going to "guess" what KB means when it comes to immigration.
there is no ATM between the plane and immigration. If they ask to see the CASH and you don't have the 20,000 baht, they'll deny entry and hold you until you pay for a new flight out. Normally only for people abusing tourist entries. But better safe than sorry.
what you've been asked has no relevance to what the law is. And it's required by law to enter as a tourist. No harm in ensuring you're following the law.
There's no such thing as a visa that "restarts" again.
A visa grants you a certain number of days when you enter Thailand, and that is your permission to stay.
A multiple-entry visa grants you those number of days every time you enter while the visa is valid.
As there are no 3 month multiple-entry visas, you are talking about a single entry visa.
That means when you enter Thailand, your visa would be used, gone, done, no longer valid. You would just have a permission to stay stamp in your passport. When you leave, that stamp will be cancelled unless you purchase a re-entry permit. If you return with a re-entry permit, you receive the same entry stamp that you left with, and it does not add any extra time. So if you left for 3 weeks and returned, you would only get a stamp for 1-2 weeks in your passport.
It's your money. You can do whatever you want with it. Just know that it will effect your next extension if you fail to meet immigration's requirements.