Uncommon issue: 3 minutes is a long time. As RogerR said, 20-30 seconds is fairly normal when you first turn on the system. After that, you should have gas immediately.
My first thoughts was the Free Flow safety valve tripped. But, you said you installed new 30# tanks. By chance do you know if the tanks were purged when first filled? This is a very important step for a new tank. Purging removes the air from the tank. If you did not, the first "gas" out of the tank is the compressed air that was trapped, during the filling process. Both tanks will have trapped air if it was not done. If that is the case, go open the other tank, and flip the tank selector to that tank, and try to light the stove/BBQ, to bleed off the compressed air. This way you will have quick access to gas in the middle of the night when the furnace is running and the auto tank transfer kicks over.
https://www.propanetankstore.com/blo...is-important-/