Three AA batteries can be found frequently in small electronic devices such as remote controls or toys. The batteries are connected in a series circuit, which generates a voltage of 4.5 volts...
Utotal = U1 + U2 + U3 = 1.5 V + 1.5 V + 1.5 V = 4.5 V
The rule also applies to series circuits that use two or more different types of batteries. That you can easily understand this with the experiment shown...
Utotal = U1 + U2 = 9.0 V + 1.5 V = 10.5 V
In practice, almost never different types of batteries are connected in a series circuit. The batteries used in the experiment have very different capacities. Why is this a problem, you find out later.