Diagnosis
The three red blinks is Ninja's error code for the safety lock not being engaged. The base doesn't think the vessel is twisted on all the way, or there's a fault in the sensor.
How to Fix It
1) Take the vessel completely off. 2) Look at the alignment arrow on the vessel and the dot on the base. Line them up perfectly. 3) Press down slightly and twist clockwise until you hear a definitive, loud CLICK. Don't be gentle. It needs to go past the initial resistance. Try the button again.
If That Didn't Work
Check the rubber seal on the bottom of the vessel and the plastic tabs on the base for any cracks, food debris, or sticky residue. Clean both areas with a damp cloth. A tiny piece of dried smoothie can block the connection.
Heads Up
If you force it on misaligned, you can break the plastic tabs. Always align the marks first. If the click sounds or feels weak, the locking mechanism might be worn.