When batteries are connected in parallel, the positive terminals are connected together and the negative terminals are connected together. This creates a single circuit with multiple paths for the current to flow. The current will flow through the path of least resistance, which in this case is the 2.5 volt battery. This means that all of the current will flow through the 2.5 volt battery and none of it will flow through the other batteries.
As a result, the voltage of the entire parallel combination will be the same as the voltage of the 2.5 volt battery.