The updated code is as below
<aura:component >
<aura:attribute name="Result" type="Integer" />
<ui:inputNumber aura:id="inputOne" label="Enter first number" />
<ui:inputNumber aura:id="inputTwo" label="Enter second number" />
<ui:inputNumber aura:id="inputThree" label="Enter third number" />
<ui:button aura:id="calculateButton" label="Calculate" press="{!c.calculate}" />
<div>
Output: <ui:outputNumber aura:id="totalValue" value="{!v.Result}" />
</div>
"c" is reserved for actions and v is reserved for attributes or values
</aura:component>